diff --git a/src/conf.zig b/src/conf.zig
index 526f5fb..3cedd48 100644
--- a/src/conf.zig
+++ b/src/conf.zig
@@ -5,4 +5,7 @@
 // included in all copies and substantial portions of the software.
 //! Values and types used by different files and that don't fit in any.
 /// Number passed to commands.
-pub const arg_type = u16;
+pub const arg_type: type = u16;
+pub const mouse_enabled: bool = true;
+pub const mouse_left_btn: c_int = 0;
+pub const mouse_right_btn: c_int = 1;
diff --git a/src/level.zig b/src/level.zig
index d08bc4c..4b22064 100644
--- a/src/level.zig
+++ b/src/level.zig
@@ -102,7 +102,7 @@ pub fn draw_selection(self: *Self, scale: u16, offset: Vec2) void {
 }
 
 /// Set the `state` of a cell at `cursor` if possible
-fn select_cell(self: *Self, cursor: Vec2, state: bool) void {
+pub fn select_cell(self: *Self, cursor: Vec2, state: bool) void {
     const target: u32 = @intCast(u32, cursor.y) * self.width + @intCast(u32, cursor.x);
     if (target < self.width * self.height and target >= 0)
         self.selection[target] = state;
@@ -118,7 +118,7 @@ fn select_clear(self: *Self, state: bool) void {
 }
 
 /// Change state of all the cells in the rectangle range.
-fn select_rect(self: *Self, rect: Rect, state: bool) void {
+pub fn select_rect(self: *Self, rect: Rect, state: bool) void {
     var cx: Rect.int_type = rect.left_x;
     while (cx <= rect.right_x) {
         defer cx += 1;
diff --git a/src/main.zig b/src/main.zig
index 4cf30c2..4fc58f0 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -8,9 +8,13 @@ const ray = @cImport({
 });
 const std = @import("std");
 const maxInt = std.math.maxInt;
+const log = std.log.info;
 
+const conf = @import("conf.zig");
 const Level = @import("level.zig");
 const Vec2 = @import("vec2.zig");
+const Rect = @import("rect.zig");
+const Mouse = @import("mouse.zig");
 const movement = @import("movement.zig");
 const scaling = @import("scaling.zig");
 const actions = @import("actions.zig");
@@ -43,6 +47,9 @@ pub fn main() !void {
     // Init scale, used by drawing code.
     var scale: scaling.scale_type = scaling.scale_default;
 
+    // Create mouse.
+    var mouse: Mouse = Mouse.init();
+
     // Create cursor.
     var cursor: Vec2 = Vec2.init(0, 0);
 
@@ -132,6 +139,66 @@ pub fn main() !void {
             }
         }
 
+        // Mouse operations.
+        if (conf.mouse_enabled) {
+            // Update position.
+            {
+                // Set mouse offset.
+                ray.SetMouseOffset(camera.x, camera.y);
+                // Set mouse scaling.
+                ray.SetMouseScale(1.0 / @intToFloat(f32, scale), 1.0 / @intToFloat(f32, scale));
+
+                mouse.pos.x = @intCast(u16, ray.GetMouseX());
+                mouse.pos.y = @intCast(u16, ray.GetMouseY());
+            }
+
+            const left_press: bool = ray.IsMouseButtonPressed(conf.mouse_left_btn);
+            const left_release: bool = ray.IsMouseButtonReleased(conf.mouse_left_btn);
+            const right_press: bool = ray.IsMouseButtonPressed(conf.mouse_right_btn);
+
+            // State machine.
+            switch (mouse.mode) {
+                // See if can switch mode.
+                .idle => {
+                    const mod_rect_sel: bool = ray.IsKeyDown(ray.KEY_LEFT_SHIFT) or
+                        ray.IsKeyDown(ray.KEY_RIGHT_SHIFT);
+                    if (left_press) { // Switch mode.
+                        cursor = mouse.pos;
+                        mouse.start_pos = mouse.pos;
+                        mouse.mode = if (mod_rect_sel)
+                            Mouse.MouseMode.rect_sel
+                        else
+                            Mouse.MouseMode.sel;
+                    }
+                },
+                // Select stuff under the cursor.
+                .sel => {
+                    cursor = mouse.pos;
+                    level.select_cell(mouse.pos, true);
+                },
+                // Rectangle selection, see `left_release` for handling.
+                .rect_sel => {},
+            }
+
+            // If left button is released, get out of current mode and apply changes if necessary.
+            if (left_release) {
+                defer mouse.mode = Mouse.MouseMode.idle;
+                defer cursor = mouse.pos;
+
+                // Select area.
+                if (mouse.mode == Mouse.MouseMode.rect_sel) {
+                    const selection = Rect.init_from_vec2(mouse.start_pos, mouse.pos);
+                    level.select_rect(selection, true);
+                }
+            }
+
+            // Reset selection on right click.
+            if (right_press) {
+                cursor = mouse.pos;
+                level.apply_selection_update(movement.move_but_dont(&cursor, 1, true));
+            }
+        }
+
         ray.BeginDrawing();
         defer ray.EndDrawing();
 
diff --git a/src/mouse.zig b/src/mouse.zig
new file mode 100644
index 0000000..f0b551a
--- /dev/null
+++ b/src/mouse.zig
@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2021 KikooDX
+// This file is part of [KBLE](https://sr.ht/~kikoodx/kble), which is
+// MIT licensed. The MIT license requires this copyright notice to be
+// included in all copies and substantial portions of the software.
+//! Mouse (eurk) logic and operations.
+const std = @import("std");
+
+const Vec2 = @import("vec2.zig");
+
+const Self = @This();
+
+pub const MouseMode = enum {
+    idle,
+    sel,
+    rect_sel,
+};
+
+mode: MouseMode, // State.
+pos: Vec2, // Where the cursor is.
+start_pos: Vec2, // Used for rectangle selection.
+
+pub fn init() Self {
+    return Self{
+        .mode = MouseMode.idle, // state
+        .pos = Vec2{ .x = 0, .y = 0 },
+        .start_pos = Vec2{ .x = 0, .y = 0 },
+    };
+}
diff --git a/src/scaling.zig b/src/scaling.zig
index 3b9d506..523cc86 100644
--- a/src/scaling.zig
+++ b/src/scaling.zig
@@ -8,7 +8,7 @@ const std = @import("std");
 const expect = std.testing.expect;
 const maxInt = std.math.maxInt;
 
-pub const scale_type = u16;
+pub const scale_type = u8;
 pub const scale_default: comptime_int = 16;
 const scale_min: comptime_int = 3;
 const scale_max: comptime_int = maxInt(scale_type);