diff --git a/src/OpenH2.Engine/RealtimeWorld.cs b/src/OpenH2.Engine/RealtimeWorld.cs
index f94aa470..91359e92 100644
--- a/src/OpenH2.Engine/RealtimeWorld.cs
+++ b/src/OpenH2.Engine/RealtimeWorld.cs
@@ -33,6 +33,7 @@ public RealtimeWorld(GameWindow window,
             Systems.Add(animationSystem);
             Systems.Add(new ScriptSystem(this, audioSystem, cameraSystem, actorSystem, animationSystem));
             Systems.Add(new RenderCollectorSystem(this, graphics));
+            Systems.Add(new MouseSelectSystem(this, window));
 
             RenderSystems.Add(new RenderPipelineSystem(this, graphics));
 
diff --git a/src/OpenH2.Engine/Systems/MouseSelectSystem.cs b/src/OpenH2.Engine/Systems/MouseSelectSystem.cs
new file mode 100644
index 00000000..f7bd8fda
--- /dev/null
+++ b/src/OpenH2.Engine/Systems/MouseSelectSystem.cs
@@ -0,0 +1,128 @@
+using OpenH2.Core.Architecture;
+using OpenH2.Engine.Stores;
+using OpenH2.Engine.Components;
+using OpenH2.Engine.Entities;
+using OpenH2.Rendering.Abstractions;
+using System.Numerics;
+using OpenTK.Windowing.Desktop;
+using System;
+
+namespace OpenH2.Engine.Systems
+{
+    public class MouseSelectSystem : WorldSystem
+    {
+
+        private readonly GameWindow window;
+        private TriggerVolume selectedVolume;
+        public MouseSelectSystem(World world, GameWindow window) : base(world)
+        {
+            selectedVolume = null;
+            this.window = window;
+        }
+
+        // Find the trigger volume that the mouse is hovering over by raycasting from the camera.
+        private TriggerVolume FindNextTriggerVolume(InputStore input_store)
+        {
+            var cameras = world.Components<CameraComponent>();
+            var cam = cameras[0];
+
+            var ViewMatrix = cam.ViewMatrix;
+            var ProjectionMatrix = cam.ProjectionMatrix;
+            var VP = ViewMatrix * ProjectionMatrix;
+
+            // Get mouse position, and convert it to projection coordinates.
+            // That is, scale [0, width) -> [-1, 1] and [0, height) -> [-1, 1].
+            var mousePos = input_store.MousePos;
+            var scaledMousePos = new Vector2((2 * mousePos.X / window.Size.X - 1), -(2 * mousePos.Y / window.Size.Y - 1));
+
+            if (!Matrix4x4.Invert(VP, out var ViewProjectionInv))
+            {
+                System.Console.WriteLine("Unable to invert view+projection matrix");
+                return null;
+            }
+
+            // Convert ray from projection space to world space.
+            var ray_origin = Vector4.Transform(new Vector4(0, 0, 0, 1), ViewProjectionInv);
+            ray_origin /= ray_origin.W;
+            var ray_offset = Vector4.Transform(new Vector4(scaledMousePos, 1, 1), ViewProjectionInv);
+            ray_offset /= ray_offset.W;
+
+            float min = float.MaxValue;
+            TriggerVolume new_volume = null;
+            foreach (var entity in this.world.Scene.Entities.Values)
+            {
+                if (entity is not TriggerVolume tv)
+                {
+                    continue;
+                }
+                // All trigger volumes have a TriggerGeometryComponent.
+                var trigGeomComp = entity.GetChildren<TriggerGeometryComponent>()[0];
+
+                var left_bottom = new Vector3(0, 0, 0);
+                var top_right = trigGeomComp.Size;
+                var entTransMat = trigGeomComp.Transform.TransformationMatrix;
+                if (!Matrix4x4.Invert(entTransMat, out var entTransMatInv))
+                {
+                    System.Console.Error.WriteLine($"Unable to invert entity transformation mat: {entTransMat} for entity {entity.FriendlyName}");
+                    return null;
+                }
+
+                // Handle non-axis-aligned triggers: transform the ray into the local space of the trigger.
+                var local_ray_origin = Vector4.Transform(ray_origin, entTransMatInv);
+                var local_ray_offset = Vector4.Transform(ray_offset, entTransMatInv);
+
+                // Now do standard axis-aligned bounding-box + ray intersection.
+                var ray_dir = local_ray_offset - local_ray_origin;
+                ray_dir /= ray_dir.Length();
+                var dirfrac = new Vector3(1.0f / ray_dir.X, 1.0f / ray_dir.Y, 1.0f / ray_dir.Z);
+
+                // Calculate the distance `t` to all 6 planes of the box.
+                float t1 = (left_bottom.X - local_ray_origin.X) * dirfrac.X;
+                float t2 = (top_right.X - local_ray_origin.X) * dirfrac.X;
+                float t3 = (left_bottom.Y - local_ray_origin.Y) * dirfrac.Y;
+                float t4 = (top_right.Y - local_ray_origin.Y) * dirfrac.Y;
+                float t5 = (left_bottom.Z - local_ray_origin.Z) * dirfrac.Z;
+                float t6 = (top_right.Z - local_ray_origin.Z) * dirfrac.Z;
+
+                float tmin = Math.Max(Math.Max(Math.Min(t1, t2), Math.Min(t3, t4)), Math.Min(t5, t6));
+                float tmax = Math.Min(Math.Min(Math.Max(t1, t2), Math.Max(t3, t4)), Math.Max(t5, t6));
+                if (tmin < 0 || tmax < 0 || tmin > tmax)
+                {
+                    // Discard the box if we are partially inside it (tmin < 0),
+                    // the box is completely behind us (tmax < 0), or the ray missed (tmin > tmax).
+                    continue;
+                }
+                if (tmin < min)
+                {
+                    // Otherwise, tmin is our total distance (in trigger-local space) to the box.
+                    // Pick the lowest one (i.e., closest the camera).
+                    min = tmin;
+                    new_volume = tv;
+                }
+            }
+            return new_volume;
+        }
+
+        public override void Update(double timestep)
+        {
+            var input_store = this.world.GetGlobalResource<InputStore>();
+            if (input_store.RightMouseDown)
+            {
+                var new_volume = FindNextTriggerVolume(input_store);
+                if (new_volume != selectedVolume)
+                {
+                    if (new_volume != null)
+                    {
+                        new_volume.ToggleSelected();
+                        System.Console.WriteLine($"[TRIG] {new_volume.FriendlyName} selected");
+                    }
+                    if (selectedVolume != null)
+                    {
+                        selectedVolume.ToggleSelected();
+                    }
+                    selectedVolume = new_volume;
+                }
+            }
+        }
+    }
+}