diff --git a/src/app.rs b/src/app.rs
index 62650cc..324f930 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -9,7 +9,7 @@ use std::sync::mpsc;
 use std::thread;
 
 use crate::exporter;
-use crate::interact::{
+use crate::interact::{MultiDragState, MultiResizeState, SelectedItem, SelectionBoxDrag, 
     AddCorridorDrag, AddDoorDrag, DragState, HoverMarker, ResizeState, draw_grid,
 };
 use crate::layout::{
@@ -57,6 +57,10 @@ pub struct DungeonApp {
     pub hover_level_idx: Option<usize>,
     pub hover_stair_idx: Option<usize>,
     pub rename_level_state: Option<(usize, String)>,
+    pub selection: Vec<SelectedItem>,
+    pub selection_box_drag: Option<SelectionBoxDrag>,
+    pub multi_drag_state: Option<MultiDragState>,
+    pub multi_resize_state: Option<MultiResizeState>,
 }
 
 impl Default for DungeonApp {
@@ -84,6 +88,10 @@ impl Default for DungeonApp {
             hover_level_idx: None,
             hover_stair_idx: None,
             rename_level_state: None,
+            selection: Vec::new(),
+            selection_box_drag: None,
+            multi_drag_state: None,
+            multi_resize_state: None,
         };
         if app.settings.composition_mode {
             app.enter_composition_layout();
@@ -308,8 +316,13 @@ impl eframe::App for DungeonApp {
                 self.delete_level(level_idx);
             }
 
-            let resizing = crate::interact::handle_resize(self, ctx, &response, &geometry);
-            if !resizing {
+            let multi_consumed = crate::interact::handle_multi_select(self, ctx, &response, &geometry);
+            let resizing = if !multi_consumed {
+                crate::interact::handle_resize(self, ctx, &response, &geometry)
+            } else {
+                false
+            };
+            if !resizing && !multi_consumed {
                 crate::interact::handle_drag(self, ctx, &response, &geometry);
             }
             crate::interact::handle_add_tool(self, &response, &geometry);
@@ -318,6 +331,10 @@ impl eframe::App for DungeonApp {
                     crate::interact::delete_at_pointer(self, pointer_pos, &geometry);
                 }
                 self.pending_delete = false;
+        self.selection.clear();
+        self.selection_box_drag = None;
+        self.multi_drag_state = None;
+        self.multi_resize_state = None;
             }
 
             if self.settings.active_level_index < self.levels.len() {
@@ -373,6 +390,8 @@ impl eframe::App for DungeonApp {
             crate::interact::draw_resize_overlay(self, &painter, &geometry);
             crate::interact::draw_corridor_hover_overlay(self, &painter, &geometry);
             crate::interact::draw_door_hover_overlay(self, &painter, &geometry);
+            crate::interact::draw_selection_highlights(self, &painter, &geometry);
+            crate::interact::draw_selection_box(self, &painter, &geometry);
         });
     }
 }
@@ -478,6 +497,10 @@ impl DungeonApp {
         self.resize_state = None;
         self.clear_hover_targets();
         self.pending_delete = false;
+        self.selection.clear();
+        self.selection_box_drag = None;
+        self.multi_drag_state = None;
+        self.multi_resize_state = None;
     }
 
     // Captures the current app state for undo history.
diff --git a/src/interact/hit_test.rs b/src/interact/hit_test.rs
index 4ff4e7f..70869e4 100644
--- a/src/interact/hit_test.rs
+++ b/src/interact/hit_test.rs
@@ -15,6 +15,9 @@ pub fn update_hover_targets(app: &mut DungeonApp, ctx: &egui::Context, geometry:
         || app.resize_state.is_some()
         || app.add_corridor_drag.is_some()
         || app.add_door_drag.is_some()
+        || app.selection_box_drag.is_some()
+        || app.multi_drag_state.is_some()
+        || app.multi_resize_state.is_some()
     {
         return;
     }
diff --git a/src/interact/mod.rs b/src/interact/mod.rs
index e12482b..8e019da 100644
--- a/src/interact/mod.rs
+++ b/src/interact/mod.rs
@@ -11,6 +11,7 @@ pub mod add_delete;
 pub mod drag;
 pub mod geometry;
 pub mod hit_test;
+pub mod multi_select;
 pub mod pathing;
 pub mod resize;
 pub mod types;
@@ -19,6 +20,7 @@ pub use add_delete::*;
 pub use drag::*;
 pub use geometry::*;
 pub use hit_test::*;
+pub use multi_select::*;
 pub use pathing::*;
 pub use resize::*;
 pub use types::*;
diff --git a/src/interact/multi_select.rs b/src/interact/multi_select.rs
new file mode 100644
index 0000000..8136d1d
--- /dev/null
+++ b/src/interact/multi_select.rs
@@ -0,0 +1,704 @@
+/*
+ * Multi-selection interaction logic.
+ *
+ * Rubber-band selection: left-click drag on empty space draws a selection box.
+ * Multi-move:  left-click drag when items are already selected (and pointer is not
+ *              on an empty space) moves all selected items together.
+ * Multi-resize: right-click drag when items are selected resizes all of them together.
+ */
+
+use super::{
+    GridGeometry, MarkerKind, MultiDragState, MultiResizeState, SelectedItem, SelectionBoxDrag,
+    corridor_drag_at_pointer, marker_at_pointer, pointer_to_grid, room_at_pointer,
+    stair_at_pointer, text_at_pointer,
+};
+use crate::app::DungeonApp;
+use crate::interact::reroute_corridors_for_room;
+use crate::ui::AddTool;
+use eframe::egui;
+use egui::{Color32, Pos2, Rect, Stroke, pos2};
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+/// Returns true when the pointer is over any interactive element.
+pub fn pointer_on_item(app: &DungeonApp, pos: Pos2, geometry: &GridGeometry) -> bool {
+    text_at_pointer(app, pos, geometry).is_some()
+        || marker_at_pointer(app, pos, geometry).is_some()
+        || stair_at_pointer(app, pos, geometry).is_some()
+        || room_at_pointer(app, pos, geometry).is_some()
+        || corridor_drag_at_pointer(app, pos, geometry).is_some()
+}
+
+/// Returns true when the pointer is over one of the currently selected items.
+pub fn pointer_on_selected_item(app: &DungeonApp, pos: Pos2, geometry: &GridGeometry) -> bool {
+    if app.selection.is_empty() {
+        return false;
+    }
+    if app.settings.active_level_index >= app.levels.len() {
+        return false;
+    }
+    let Some((grid_x, grid_y)) = pointer_to_grid(pos, geometry) else {
+        return false;
+    };
+    let cell = (grid_x.floor() as usize, grid_y.floor() as usize);
+    let layout = &app.levels[app.settings.active_level_index];
+
+    for item in &app.selection {
+        match item {
+            SelectedItem::Room(idx) => {
+                if let Some(r) = layout.rooms.get(*idx) {
+                    if cell.0 >= r.x
+                        && cell.0 < r.x + r.width
+                        && cell.1 >= r.y
+                        && cell.1 < r.y + r.height
+                    {
+                        return true;
+                    }
+                }
+            }
+            SelectedItem::Text(idx) => {
+                if let Some(l) = layout.text_labels.get(*idx) {
+                    if l.cell == cell {
+                        return true;
+                    }
+                }
+            }
+            SelectedItem::Staircase(idx) => {
+                if let Some(s) = layout.stairs.get(*idx) {
+                    if cell.0 >= s.cell.0
+                        && cell.0 < s.cell.0 + s.width
+                        && cell.1 >= s.cell.1
+                        && cell.1 < s.cell.1 + s.height
+                    {
+                        return true;
+                    }
+                }
+            }
+            SelectedItem::Marker(kind, idx) => {
+                let markers = match kind {
+                    MarkerKind::Start => &layout.start_markers,
+                    MarkerKind::End => &layout.end_markers,
+                    MarkerKind::Trap => &layout.trap_markers,
+                    MarkerKind::Monster => &layout.monster_markers,
+                };
+                if let Some(m) = markers.get(*idx) {
+                    if cell.0 >= m.cell.0
+                        && cell.0 < m.cell.0 + m.size
+                        && cell.1 >= m.cell.1
+                        && cell.1 < m.cell.1 + m.size
+                    {
+                        return true;
+                    }
+                }
+            }
+        }
+    }
+    false
+}
+
+/// Returns the top-left grid cell of a selected item.
+fn item_origin(app: &DungeonApp, item: &SelectedItem) -> Option<(isize, isize)> {
+    if app.settings.active_level_index >= app.levels.len() {
+        return None;
+    }
+    let layout = &app.levels[app.settings.active_level_index];
+    match item {
+        SelectedItem::Room(idx) => layout
+            .rooms
+            .get(*idx)
+            .map(|r| (r.x as isize, r.y as isize)),
+        SelectedItem::Text(idx) => layout
+            .text_labels
+            .get(*idx)
+            .map(|l| (l.cell.0 as isize, l.cell.1 as isize)),
+        SelectedItem::Staircase(idx) => layout
+            .stairs
+            .get(*idx)
+            .map(|s| (s.cell.0 as isize, s.cell.1 as isize)),
+        SelectedItem::Marker(kind, idx) => {
+            let markers = match kind {
+                MarkerKind::Start => &layout.start_markers,
+                MarkerKind::End => &layout.end_markers,
+                MarkerKind::Trap => &layout.trap_markers,
+                MarkerKind::Monster => &layout.monster_markers,
+            };
+            markers.get(*idx).map(|m| (m.cell.0 as isize, m.cell.1 as isize))
+        }
+    }
+}
+
+/// Returns (x, y, width, height) for resize origins. For text, width = font_size as isize, height = 0.
+fn item_size_origin(app: &DungeonApp, item: &SelectedItem) -> Option<(isize, isize, isize, isize)> {
+    if app.settings.active_level_index >= app.levels.len() {
+        return None;
+    }
+    let layout = &app.levels[app.settings.active_level_index];
+    match item {
+        SelectedItem::Room(idx) => layout.rooms.get(*idx).map(|r| {
+            (r.x as isize, r.y as isize, r.width as isize, r.height as isize)
+        }),
+        SelectedItem::Staircase(idx) => layout.stairs.get(*idx).map(|s| {
+            (s.cell.0 as isize, s.cell.1 as isize, s.width as isize, s.height as isize)
+        }),
+        SelectedItem::Text(idx) => layout.text_labels.get(*idx).map(|l| {
+            (l.cell.0 as isize, l.cell.1 as isize, l.font_size as isize, 0)
+        }),
+        SelectedItem::Marker(kind, idx) => {
+            let markers = match kind {
+                MarkerKind::Start => &layout.start_markers,
+                MarkerKind::End => &layout.end_markers,
+                MarkerKind::Trap => &layout.trap_markers,
+                MarkerKind::Monster => &layout.monster_markers,
+            };
+            markers.get(*idx).map(|m| {
+                (m.cell.0 as isize, m.cell.1 as isize, m.size as isize, m.size as isize)
+            })
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Selection box
+// ---------------------------------------------------------------------------
+
+/// Start a rubber-band selection drag.
+pub fn start_selection_box(app: &mut DungeonApp, pos: Pos2, geometry: &GridGeometry) {
+    let Some((gx, gy)) = pointer_to_grid(pos, geometry) else { return };
+    app.selection.clear();
+    app.selection_box_drag = Some(SelectionBoxDrag {
+        start_cell: (gx, gy),
+        current_cell: (gx, gy),
+    });
+}
+
+/// Update the rubber-band selection rectangle and compute the new selection set.
+pub fn update_selection_box(app: &mut DungeonApp, pos: Pos2, geometry: &GridGeometry) {
+    let Some((gx, gy)) = pointer_to_grid(pos, geometry) else { return };
+    if let Some(ref mut drag) = app.selection_box_drag {
+        drag.current_cell = (gx, gy);
+    } else {
+        return;
+    }
+
+    if app.settings.active_level_index >= app.levels.len() {
+        return;
+    }
+
+    let (sx, sy, cx, cy) = {
+        let drag = app.selection_box_drag.as_ref().unwrap();
+        (drag.start_cell.0, drag.start_cell.1, drag.current_cell.0, drag.current_cell.1)
+    };
+
+    let min_x = sx.min(cx).floor() as usize;
+    let min_y = sy.min(cy).floor() as usize;
+    let max_x = sx.max(cx).ceil() as usize;
+    let max_y = sy.max(cy).ceil() as usize;
+
+    let layout = &app.levels[app.settings.active_level_index];
+    let mut selection = Vec::new();
+
+    for (idx, room) in layout.rooms.iter().enumerate() {
+        // Room overlaps selection box if they intersect.
+        if room.x < max_x
+            && room.x + room.width > min_x
+            && room.y < max_y
+            && room.y + room.height > min_y
+        {
+            selection.push(SelectedItem::Room(idx));
+        }
+    }
+
+    for (idx, label) in layout.text_labels.iter().enumerate() {
+        if label.cell.0 >= min_x
+            && label.cell.0 < max_x
+            && label.cell.1 >= min_y
+            && label.cell.1 < max_y
+        {
+            selection.push(SelectedItem::Text(idx));
+        }
+    }
+
+    for (idx, stair) in layout.stairs.iter().enumerate() {
+        if stair.cell.0 < max_x
+            && stair.cell.0 + stair.width > min_x
+            && stair.cell.1 < max_y
+            && stair.cell.1 + stair.height > min_y
+        {
+            selection.push(SelectedItem::Staircase(idx));
+        }
+    }
+
+    let add_markers = |markers: &Vec<crate::layout::AreaMarker>,
+                       kind: MarkerKind,
+                       selection: &mut Vec<SelectedItem>| {
+        for (idx, m) in markers.iter().enumerate() {
+            if m.cell.0 < max_x
+                && m.cell.0 + m.size > min_x
+                && m.cell.1 < max_y
+                && m.cell.1 + m.size > min_y
+            {
+                selection.push(SelectedItem::Marker(kind, idx));
+            }
+        }
+    };
+
+    add_markers(&layout.start_markers.clone(), MarkerKind::Start, &mut selection);
+    add_markers(&layout.end_markers.clone(), MarkerKind::End, &mut selection);
+    add_markers(&layout.trap_markers.clone(), MarkerKind::Trap, &mut selection);
+    add_markers(&layout.monster_markers.clone(), MarkerKind::Monster, &mut selection);
+
+    app.selection = selection;
+}
+
+/// Finish the rubber-band drag (keep current selection).
+pub fn finish_selection_box(app: &mut DungeonApp) {
+    app.selection_box_drag = None;
+    // If nothing ended up selected, clear to stay clean.
+    // (keep the selection so the user can immediately use it)
+}
+
+// ---------------------------------------------------------------------------
+// Multi-drag (left button move)
+// ---------------------------------------------------------------------------
+
+/// Start a multi-item drag.
+pub fn start_multi_drag(app: &mut DungeonApp, pos: Pos2, geometry: &GridGeometry) {
+    let Some((gx, gy)) = pointer_to_grid(pos, geometry) else { return };
+
+    let origins: Vec<(SelectedItem, (isize, isize))> = app
+        .selection
+        .iter()
+        .filter_map(|item| {
+            let origin = item_origin(app, item)?;
+            Some((item.clone(), origin))
+        })
+        .collect();
+
+    if origins.is_empty() {
+        return;
+    }
+
+    app.push_undo_snapshot();
+    app.multi_drag_state = Some(MultiDragState {
+        start_grid: (gx, gy),
+        origins,
+    });
+}
+
+/// Apply multi-item movement based on current pointer position.
+pub fn update_multi_drag(app: &mut DungeonApp, pos: Pos2, geometry: &GridGeometry) {
+    let Some((gx, gy)) = pointer_to_grid(pos, geometry) else { return };
+    let Some(state) = app.multi_drag_state.clone() else { return };
+
+    let dx = (gx - state.start_grid.0).round() as isize;
+    let dy = (gy - state.start_grid.1).round() as isize;
+
+    if app.settings.active_level_index >= app.levels.len() {
+        return;
+    }
+
+    let cols = app.settings.cols as isize;
+    let rows = app.settings.rows as isize;
+
+    for (item, (ox, oy)) in &state.origins {
+        let new_x = (ox + dx).max(0);
+        let new_y = (oy + dy).max(0);
+
+        match item {
+            SelectedItem::Room(idx) => {
+                let layout = &mut app.levels[app.settings.active_level_index];
+                if let Some(room) = layout.rooms.get_mut(*idx) {
+                    let max_x = (cols - room.width as isize).max(0);
+                    let max_y = (rows - room.height as isize).max(0);
+                    room.x = new_x.min(max_x) as usize;
+                    room.y = new_y.min(max_y) as usize;
+                }
+            }
+            SelectedItem::Text(idx) => {
+                let layout = &mut app.levels[app.settings.active_level_index];
+                if let Some(label) = layout.text_labels.get_mut(*idx) {
+                    label.cell = (
+                        new_x.min(cols - 1).max(0) as usize,
+                        new_y.min(rows - 1).max(0) as usize,
+                    );
+                }
+            }
+            SelectedItem::Staircase(idx) => {
+                let layout = &mut app.levels[app.settings.active_level_index];
+                if let Some(stair) = layout.stairs.get_mut(*idx) {
+                    let max_x = (cols - stair.width as isize).max(0);
+                    let max_y = (rows - stair.height as isize).max(0);
+                    stair.cell = (new_x.min(max_x) as usize, new_y.min(max_y) as usize);
+                }
+            }
+            SelectedItem::Marker(kind, idx) => {
+                let layout = &mut app.levels[app.settings.active_level_index];
+                let markers = match kind {
+                    MarkerKind::Start => &mut layout.start_markers,
+                    MarkerKind::End => &mut layout.end_markers,
+                    MarkerKind::Trap => &mut layout.trap_markers,
+                    MarkerKind::Monster => &mut layout.monster_markers,
+                };
+                if let Some(m) = markers.get_mut(*idx) {
+                    let max_x = (cols - m.size as isize).max(0);
+                    let max_y = (rows - m.size as isize).max(0);
+                    m.cell = (new_x.min(max_x) as usize, new_y.min(max_y) as usize);
+                }
+            }
+        }
+    }
+
+    // Reroute corridors for all moved rooms.
+    let moved_rooms: Vec<usize> = state
+        .origins
+        .iter()
+        .filter_map(|(item, _)| {
+            if let SelectedItem::Room(idx) = item { Some(*idx) } else { None }
+        })
+        .collect();
+    for room_idx in moved_rooms {
+        reroute_corridors_for_room(app, room_idx);
+    }
+    app.refresh_doors();
+}
+
+/// Finish multi-drag.
+pub fn finish_multi_drag(app: &mut DungeonApp) {
+    app.multi_drag_state = None;
+}
+
+// ---------------------------------------------------------------------------
+// Multi-resize (right button)
+// ---------------------------------------------------------------------------
+
+/// Start a multi-item resize.
+pub fn start_multi_resize(app: &mut DungeonApp, pos: Pos2, geometry: &GridGeometry) {
+    let Some((gx, gy)) = pointer_to_grid(pos, geometry) else { return };
+
+    let origins: Vec<(SelectedItem, (isize, isize, isize, isize))> = app
+        .selection
+        .iter()
+        .filter_map(|item| {
+            let origin = item_size_origin(app, item)?;
+            Some((item.clone(), origin))
+        })
+        .collect();
+
+    if origins.is_empty() {
+        return;
+    }
+
+    app.push_undo_snapshot();
+    app.multi_resize_state = Some(MultiResizeState {
+        start_grid: (gx, gy),
+        origins,
+    });
+}
+
+/// Apply multi-item resize based on current pointer position.
+/// Scale factor is driven by the mouse delta: moving right/down grows items,
+/// moving left/up shrinks them. The delta is applied as an integer cell offset
+/// to width and height (rooms/stairs grow from bottom-right corner; text scales font).
+pub fn update_multi_resize(app: &mut DungeonApp, pos: Pos2, geometry: &GridGeometry) {
+    let Some((gx, gy)) = pointer_to_grid(pos, geometry) else { return };
+    let Some(state) = app.multi_resize_state.clone() else { return };
+
+    let dx = (gx - state.start_grid.0).round() as isize;
+    let dy = (gy - state.start_grid.1).round() as isize;
+    // Use the larger of dx/dy as a single resize delta so all items scale uniformly.
+    let delta = if dx.abs() >= dy.abs() { dx } else { dy };
+
+    if app.settings.active_level_index >= app.levels.len() {
+        return;
+    }
+
+    let cols = app.settings.cols as isize;
+    let rows = app.settings.rows as isize;
+
+    for (item, (ox, oy, ow, oh)) in &state.origins {
+        match item {
+            SelectedItem::Room(idx) => {
+                let new_w = (*ow + delta).max(1);
+                let new_h = (*oh + delta).max(1);
+                let layout = &mut app.levels[app.settings.active_level_index];
+                if let Some(room) = layout.rooms.get_mut(*idx) {
+                    if ox + new_w <= cols && oy + new_h <= rows {
+                        room.width = new_w as usize;
+                        room.height = new_h as usize;
+                    }
+                }
+            }
+            SelectedItem::Staircase(idx) => {
+                let new_w = (*ow + delta).max(1);
+                let new_h = (*oh + delta).max(1);
+                let layout = &mut app.levels[app.settings.active_level_index];
+                if let Some(stair) = layout.stairs.get_mut(*idx) {
+                    if ox + new_w <= cols && oy + new_h <= rows {
+                        stair.width = new_w as usize;
+                        stair.height = new_h as usize;
+                    }
+                }
+            }
+            SelectedItem::Text(idx) => {
+                // ow stores the original font_size for text labels.
+                let new_size = (*ow + delta * 4).clamp(8, 96) as u16;
+                let layout = &mut app.levels[app.settings.active_level_index];
+                if let Some(label) = layout.text_labels.get_mut(*idx) {
+                    label.font_size = new_size;
+                }
+            }
+            SelectedItem::Marker(kind, idx) => {
+                let new_size = (*ow + delta).max(1) as usize;
+                let layout = &mut app.levels[app.settings.active_level_index];
+                let markers = match kind {
+                    MarkerKind::Start => &mut layout.start_markers,
+                    MarkerKind::End => &mut layout.end_markers,
+                    MarkerKind::Trap => &mut layout.trap_markers,
+                    MarkerKind::Monster => &mut layout.monster_markers,
+                };
+                if let Some(m) = markers.get_mut(*idx) {
+                    let max_x = (cols - new_size as isize).max(0);
+                    let max_y = (rows - new_size as isize).max(0);
+                    if *ox <= max_x && *oy <= max_y {
+                        m.size = new_size;
+                    }
+                }
+            }
+        }
+    }
+
+    // Reroute corridors for resized rooms.
+    let resized_rooms: Vec<usize> = state
+        .origins
+        .iter()
+        .filter_map(|(item, _)| {
+            if let SelectedItem::Room(idx) = item { Some(*idx) } else { None }
+        })
+        .collect();
+    for room_idx in resized_rooms {
+        reroute_corridors_for_room(app, room_idx);
+    }
+    app.refresh_doors();
+}
+
+/// Finish multi-resize.
+pub fn finish_multi_resize(app: &mut DungeonApp) {
+    app.multi_resize_state = None;
+}
+
+// ---------------------------------------------------------------------------
+// Master handler (called from app.rs update loop)
+// ---------------------------------------------------------------------------
+
+/// Central handler for all multi-selection interactions.
+/// Returns true if it consumed the input so the single-item handlers are skipped.
+pub fn handle_multi_select(
+    app: &mut DungeonApp,
+    ctx: &egui::Context,
+    response: &egui::Response,
+    geometry: &GridGeometry,
+) -> bool {
+    if app.settings.add_tool != AddTool::None {
+        return false;
+    }
+
+    // --- Right click: multi-resize ---
+    if ctx.input(|i| i.pointer.secondary_pressed())
+        && !app.selection.is_empty()
+        && app.multi_resize_state.is_none()
+    {
+        if let Some(pos) = response.interact_pointer_pos() {
+            start_multi_resize(app, pos, geometry);
+            return true;
+        }
+    }
+
+    if app.multi_resize_state.is_some() {
+        if response.dragged_by(egui::PointerButton::Secondary)
+            && ctx.input(|i| i.pointer.secondary_down())
+        {
+            if let Some(pos) = response.interact_pointer_pos() {
+                update_multi_resize(app, pos, geometry);
+            }
+        }
+        if ctx.input(|i| i.pointer.button_released(egui::PointerButton::Secondary)) {
+            finish_multi_resize(app);
+        }
+        return true;
+    }
+
+    // --- Left click on empty space: rubber-band select ---
+    if response.drag_started()
+        && ctx.input(|i| i.pointer.primary_down())
+        && app.multi_drag_state.is_none()
+        && app.selection_box_drag.is_none()
+    {
+        if let Some(pos) = response.interact_pointer_pos() {
+            let on_item = pointer_on_item(app, pos, geometry);
+            let on_selected = pointer_on_selected_item(app, pos, geometry);
+
+            if on_selected && !app.selection.is_empty() {
+                // Start moving the selection.
+                start_multi_drag(app, pos, geometry);
+                return true;
+            } else if !on_item {
+                // Start rubber-band selection.
+                start_selection_box(app, pos, geometry);
+                return true;
+            }
+            // on an unselected item — fall through to single-item handlers.
+        }
+    }
+
+    if app.selection_box_drag.is_some() {
+        if response.dragged() {
+            if let Some(pos) = response.interact_pointer_pos() {
+                update_selection_box(app, pos, geometry);
+            }
+        }
+        if response.drag_stopped() || !ctx.input(|i| i.pointer.primary_down()) {
+            finish_selection_box(app);
+        }
+        return true;
+    }
+
+    if app.multi_drag_state.is_some() {
+        if response.dragged() && ctx.input(|i| i.pointer.primary_down()) {
+            if let Some(pos) = response.interact_pointer_pos() {
+                update_multi_drag(app, pos, geometry);
+            }
+        }
+        if response.drag_stopped() || !ctx.input(|i| i.pointer.primary_down()) {
+            finish_multi_drag(app);
+        }
+        return true;
+    }
+
+    // Clear selection on plain click on empty space.
+    if response.clicked() {
+        if let Some(pos) = response.interact_pointer_pos() {
+            if !pointer_on_item(app, pos, geometry) {
+                app.selection.clear();
+            }
+        }
+    }
+
+    false
+}
+
+// ---------------------------------------------------------------------------
+// Drawing
+// ---------------------------------------------------------------------------
+
+/// Draws the rubber-band selection box while dragging.
+pub fn draw_selection_box(app: &DungeonApp, painter: &egui::Painter, geometry: &GridGeometry) {
+    let Some(drag) = &app.selection_box_drag else { return };
+
+    let to_screen = |gx: f32, gy: f32| -> Pos2 {
+        pos2(
+            geometry.rect.left() + gx * geometry.cell_size,
+            geometry.rect.top() + gy * geometry.cell_size,
+        )
+    };
+
+    let tl = to_screen(
+        drag.start_cell.0.min(drag.current_cell.0),
+        drag.start_cell.1.min(drag.current_cell.1),
+    );
+    let br = to_screen(
+        drag.start_cell.0.max(drag.current_cell.0),
+        drag.start_cell.1.max(drag.current_cell.1),
+    );
+
+    let rect = Rect::from_min_max(tl, br);
+    painter.rect_filled(rect, 0.0, Color32::from_rgba_unmultiplied(100, 160, 255, 30));
+    painter.rect_stroke(
+        rect,
+        0.0,
+        Stroke::new(1.5, Color32::from_rgba_unmultiplied(100, 160, 255, 200)),
+        egui::StrokeKind::Middle,
+    );
+}
+
+/// Draws highlight outlines around all selected items.
+pub fn draw_selection_highlights(
+    app: &DungeonApp,
+    painter: &egui::Painter,
+    geometry: &GridGeometry,
+) {
+    if app.selection.is_empty() {
+        return;
+    }
+    if app.settings.active_level_index >= app.levels.len() {
+        return;
+    }
+    let layout = &app.levels[app.settings.active_level_index];
+    let color = Color32::from_rgba_unmultiplied(100, 180, 255, 220);
+    let stroke = Stroke::new(2.0, color);
+
+    for item in &app.selection {
+        match item {
+            SelectedItem::Room(idx) => {
+                if let Some(r) = layout.rooms.get(*idx) {
+                    let left = geometry.rect.left() + r.x as f32 * geometry.cell_size;
+                    let top = geometry.rect.top() + r.y as f32 * geometry.cell_size;
+                    let rect = Rect::from_min_size(
+                        pos2(left, top),
+                        egui::vec2(
+                            r.width as f32 * geometry.cell_size,
+                            r.height as f32 * geometry.cell_size,
+                        ),
+                    );
+                    painter.rect_stroke(rect, 0.0, stroke, egui::StrokeKind::Middle);
+                }
+            }
+            SelectedItem::Staircase(idx) => {
+                if let Some(s) = layout.stairs.get(*idx) {
+                    let left = geometry.rect.left() + s.cell.0 as f32 * geometry.cell_size;
+                    let top = geometry.rect.top() + s.cell.1 as f32 * geometry.cell_size;
+                    let rect = Rect::from_min_size(
+                        pos2(left, top),
+                        egui::vec2(
+                            s.width as f32 * geometry.cell_size,
+                            s.height as f32 * geometry.cell_size,
+                        ),
+                    );
+                    painter.rect_stroke(rect, 0.0, stroke, egui::StrokeKind::Middle);
+                }
+            }
+            SelectedItem::Text(idx) => {
+                if let Some(l) = layout.text_labels.get(*idx) {
+                    let center = pos2(
+                        geometry.rect.left() + (l.cell.0 as f32 + 0.5) * geometry.cell_size,
+                        geometry.rect.top() + (l.cell.1 as f32 + 0.5) * geometry.cell_size,
+                    );
+                    let half = geometry.cell_size * 0.5;
+                    let rect = Rect::from_center_size(center, egui::vec2(half * 2.0, half * 2.0));
+                    painter.rect_stroke(rect, 0.0, stroke, egui::StrokeKind::Middle);
+                }
+            }
+            SelectedItem::Marker(kind, idx) => {
+                let markers = match kind {
+                    MarkerKind::Start => &layout.start_markers,
+                    MarkerKind::End => &layout.end_markers,
+                    MarkerKind::Trap => &layout.trap_markers,
+                    MarkerKind::Monster => &layout.monster_markers,
+                };
+                if let Some(m) = markers.get(*idx) {
+                    let left = geometry.rect.left() + m.cell.0 as f32 * geometry.cell_size;
+                    let top = geometry.rect.top() + m.cell.1 as f32 * geometry.cell_size;
+                    let rect = Rect::from_min_size(
+                        pos2(left, top),
+                        egui::vec2(
+                            m.size as f32 * geometry.cell_size,
+                            m.size as f32 * geometry.cell_size,
+                        ),
+                    );
+                    painter.rect_stroke(rect, 0.0, stroke, egui::StrokeKind::Middle);
+                }
+            }
+        }
+    }
+}
diff --git a/src/interact/types.rs b/src/interact/types.rs
index efad46b..8801e58 100644
--- a/src/interact/types.rs
+++ b/src/interact/types.rs
@@ -53,7 +53,7 @@ pub enum ResizeCorner {
     BottomRight,
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum MarkerKind {
     Start,
     End,
@@ -116,3 +116,43 @@ pub enum ResizeState {
     Text(TextResizeState),
     Staircase(StaircaseResizeState),
 }
+
+// ---- Multi-selection types ----
+
+/// Identifies a single selectable item in the dungeon layout.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub enum SelectedItem {
+    Room(usize),
+    Text(usize),
+    Staircase(usize),
+    Marker(MarkerKind, usize),
+}
+
+/// State while the user is rubber-band selecting (left-click drag on empty space).
+#[derive(Debug, Clone)]
+pub struct SelectionBoxDrag {
+    /// Grid-space start corner of the selection rectangle.
+    pub start_cell: (f32, f32),
+    /// Grid-space current corner of the selection rectangle (tracks the pointer).
+    pub current_cell: (f32, f32),
+}
+
+/// State while dragging a multi-selection with the left mouse button.
+#[derive(Debug, Clone)]
+pub struct MultiDragState {
+    /// Grid-space position when the drag started (used to compute delta).
+    pub start_grid: (f32, f32),
+    /// Snapshot of each selected item's original position at drag start.
+    pub origins: Vec<(SelectedItem, (isize, isize))>,
+}
+
+/// State while resizing a multi-selection with the right mouse button.
+#[derive(Debug, Clone)]
+pub struct MultiResizeState {
+    /// Grid-space position when the resize started.
+    pub start_grid: (f32, f32),
+    /// Snapshot of each selected item's original size at resize start.
+    /// For rooms/staircases: (x, y, width, height).
+    /// For text: (cell_x, cell_y, font_size as width, 0).
+    pub origins: Vec<(SelectedItem, (isize, isize, isize, isize))>,
+}