diff --git a/scratches/bootstrap.clj b/scratches/bootstrap.clj
index 0d9c783..552c5ee 100644
--- a/scratches/bootstrap.clj
+++ b/scratches/bootstrap.clj
@@ -1,16 +1,17 @@
 ;; you should be in age-of-sail.core>
 
-(def my-ship (ref {:position [0. 0.]
-                   :name "Virginia Woolfe"
+(def virginia-woolfe (ref {:position [0. 0.]
+                           :name "Virginia Woolfe"
+                           :heading (normalize [1.0 1.0])
+                           :slots [{:type :downwind-sail :length 2 :furl 1.0}]
+                           :velocity [1. 0.]}))
+
+(def revenge (ref {:position [0. 0.]
+                   :name "Revenge"
                    :heading (normalize [1.0 1.0])
                    :slots [{:type :downwind-sail :length 2 :furl 1.0}]
                    :velocity [1. 0.]}))
 
-(reset! ships [my-ship])
+(reset! ships [virginia-woolfe revenge])
 
-(start-program)
-@my-ship
-(stop-program)
-
-(show-ui)
 @tracked-ship
diff --git a/src/age_of_sail/core.clj b/src/age_of_sail/core.clj
index 2fcf84d..5a2ebb2 100644
--- a/src/age_of_sail/core.clj
+++ b/src/age_of_sail/core.clj
@@ -7,8 +7,10 @@
 (def tickrate 100)
 (def hardcoded-wind [0.1 3.0]) ;; A strong easternly wind!
 (defonce ships (atom []))
-(defonce program (atom nil))
 
+(defn list-ships
+  [ships]
+  (map #(:name @%) ships))
 (defn find-ship
   [ships name]
   (some #(when (= name (:name @%)) %) ships))
@@ -44,39 +46,60 @@
   (doseq [ship ships]
     (-> ship (physics-step hardcoded-wind))))
 
-(defn stop-program
+;; Simulation controls
+(defonce program (atom :stopped))
+(defn game-loop
   []
-  (when @program
-    (reset! @program false)))
+  (while (#{:running :paused} @program)
+    (when (= :running @program)
+      (tick @ships))
+    (Thread/sleep (quot 1000 tickrate)))
+  (when-not (compare-and-set! program :killed :stopped)
+    (throw "Error: tried to stop a program that wasn't killed!")))
+
+(defn pause-program
+  []
+  (compare-and-set! program :running :paused))
+
+(defn kill-program
+  []
+  (compare-and-set! program :running :paused)
+  (compare-and-set! program :paused :killed))
 
 (defn start-program
   []
-  (stop-program)
-  (let [thread-continue (atom true)]
-    (.start
-     (Thread.
-      (fn []
-        (while @thread-continue
-          (tick @ships)
-          (Thread/sleep (quot 1000 tickrate))))))
-    (reset! program thread-continue)))
+  (when (= (first (reset-vals! program :running)) :stopped)
+    (.start (Thread. game-loop))))
 
 ;; UI
+(defn ignore-args
+  [f]
+  (fn [& _] (f)))
+
 (def tracked-ship (atom nil))
 
-(defn ship-start-stop
+(defn simulation-controls
   []
-  (letfn [(indicator [running] (if running "Stop" "Start"))
-          (toggle-prgm [_] (if @@program (stop-program) (start-program)))]
-    (let [btn (button :text (indicator @@program))]
-      (b/bind @program (b/transform indicator) (b/property btn :text))
-      (listen btn :mouse-clicked toggle-prgm)
-      btn)))
+  (let [start-button (button :text "Start" :listen [:mouse-clicked (ignore-args start-program)])
+        pause-button (button :text "Pause" :listen [:mouse-clicked (ignore-args pause-program)])
+        kill-button (button :text "Kill" :listen [:mouse-clicked (ignore-args kill-program)])]
+    (letfn [(start-enabled? [state] (contains? #{:stopped :paused} state))
+            (pause-enabled? [state] (= :running state))
+            (kill-enabled? [state] (contains? #{:running :paused} state))]
+      (b/bind program (b/tee
+                       (b/bind (b/transform start-enabled?) (b/property start-button :enabled?))
+                       (b/bind (b/transform pause-enabled?) (b/property pause-button :enabled?))
+                       (b/bind (b/transform kill-enabled?) (b/property kill-button :enabled?))))
+      (config! start-button :enabled? (start-enabled? @program))
+      (config! pause-button :enabled? (pause-enabled? @program))
+      (config! kill-button :enabled? (kill-enabled? @program))
+      (horizontal-panel :items [start-button pause-button kill-button]))))
 
 (defn ship-chooser
   []
-  (let [name (text :columns 20)]
-    (b/bind (.getDocument name) (b/transform #(find-ship @ships %)) tracked-ship)
+  (let [name (combobox :model (list-ships @ships))]
+    (b/bind ships (b/transform list-ships) (b/property name :model))
+    (b/bind (b/selection name) (b/transform #(find-ship @ships %)) tracked-ship)
     (flow-panel :items ["Ship Name" name])))
 
 (defn format-position
@@ -96,13 +119,12 @@
 (defn show-ui
   []
   (let [root (frame :title "Age of Sail" :content (vertical-panel))
-        start-stop (ship-start-stop)
         chooser (ship-chooser)
         info (ship-info)]
     (b/bind tracked-ship (b/transform boolean) (b/tee (b/property info :visible?)
                                                       (b/b-do [_] (pack! root))))
     (doto root
-      (add! start-stop)
+      (add! (simulation-controls))
       (add! chooser)
       (add! info)
       pack!