diff --git a/src/main.rs b/src/main.rs
index 645b590..9de0821 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -6,6 +6,7 @@ mod web;
 mod render;
 mod camera;
 mod params;
+mod ui;
 
 fn main() {
     // On web, abort startup if WebGPU isn't available and show a message.
diff --git a/src/render/plugin.rs b/src/render/plugin.rs
index 637eae6..fd3016d 100644
--- a/src/render/plugin.rs
+++ b/src/render/plugin.rs
@@ -10,8 +10,15 @@ impl Plugin for BlackHolePlugin {
         app.init_resource::<crate::camera::OrbitCamera>()
             .init_resource::<crate::params::BlackHoleParams>()
             .add_plugins(Material2dPlugin::<BlackHoleMaterial>::default())
+            .add_plugins(bevy_egui::EguiPlugin::default())
             .add_systems(Startup, spawn_fullscreen_quad)
-            .add_systems(Update, (crate::camera::orbit_controller, mirror_params));
+            .add_systems(
+                Update,
+                (crate::camera::orbit_controller, mirror_params),
+            )
+            // bevy_egui 0.41 requires UI systems to run inside the egui context
+            // pass (fonts/ctx are initialized there); placing them in Update panics.
+            .add_systems(bevy_egui::EguiPrimaryContextPass, crate::ui::ui_system);
     }
 }
 
diff --git a/src/ui.rs b/src/ui.rs
new file mode 100644
index 0000000..7a74539
--- /dev/null
+++ b/src/ui.rs
@@ -0,0 +1,17 @@
+use bevy::prelude::*;
+use bevy_egui::egui;
+
+pub fn ui_system(
+    mut contexts: bevy_egui::EguiContexts,
+    mut params: ResMut<crate::params::BlackHoleParams>,
+) {
+    if let Ok(ctx) = contexts.ctx_mut() {
+        egui::Window::new("Controls").show(ctx, |ui| {
+            ui.add(
+                egui::Slider::new(&mut params.disk_brightness, 0.0..=3.0)
+                    .text("Disk brightness"),
+            );
+            ui.add(egui::Slider::new(&mut params.steps, 50..=600).text("Steps"));
+        });
+    }
+}