diff --git a/src/tracker/gui.js b/src/tracker/gui.js
index 06afc07..dfc9fba 100644
--- a/src/tracker/gui.js
+++ b/src/tracker/gui.js
@@ -1 +1,109 @@
-/* gui.js */
+var GUI = (function(){
+  var controller;
+  
+  var root = {
+    showController: function(){
+      controller = {};
+      
+      // styles
+      
+      controller.styles = DOM.createStyle([
+        ".app, .connecting { bottom: 48px !important; }",
+        "#dht-ctrl { position: absolute; bottom: 0; width: 100%; height: 48px; background-color: #fff; }",
+        "#dht-ctrl button { height: 32px; margin: 8px 0 8px 8px; font-size: 18px; padding: 0 12px; background-color: #adf; }",
+        "#dht-ctrl button:disabled { background-color: #d0d0d0; cursor: default; }",
+        "#dht-ctrl p { display: inline-block; margin: 14px 12px; }",
+        "#dht-ctrl input { display: none; }"
+      ]);
+      
+      // main
+      
+      controller.ele = DOM.createElement("div", document.body);
+      controller.ele.id = "dht-ctrl";
+      
+      controller.ele.innerHTML = [
+        "<button id='dht-ctrl-upload'>Upload Previous File</button>",
+        "<button id='dht-ctrl-track'>Start Tracking</button>",
+        "<button id='dht-ctrl-download'>Download</button>",
+        "<button id='dht-ctrl-reset'>Reset</button>",
+        "<p id='dht-ctrl-status'></p>",
+        "<input id='dht-ctrl-upload-input' type='file' multiple>"
+      ].join("");
+      
+      // elements
+      
+      controller.ui = {
+        btnUpload: DOM.id("dht-ctrl-upload"),
+        btnToggleTracking: DOM.id("dht-ctrl-track"),
+        btnDownload: DOM.id("dht-ctrl-download"),
+        btnReset: DOM.id("dht-ctrl-reset"),
+        textStatus: DOM.id("dht-ctrl-status"),
+        inputUpload: DOM.id("dht-ctrl-upload-input")
+      };
+      
+      // events
+      
+      controller.ui.btnUpload.addEventListener("click", () => {
+        controller.ui.inputUpload.click();
+      });
+      
+      controller.ui.btnToggleTracking.addEventListener("click", () => {
+        STATE.toggleTracking();
+      });
+      
+      controller.ui.btnDownload.addEventListener("click", () => {
+        STATE.downloadSavefile();
+      });
+      
+      controller.ui.btnReset.addEventListener("click", () => {
+        STATE.resetState();
+      });
+      
+      controller.ui.inputUpload.addEventListener("change", () => {
+        for(var file of controller.ui.inputUpload.files){
+          var reader = new FileReader();
+          
+          reader.onload = function(){
+            var obj = {};
+
+            try{
+              obj = JSON.parse(reader.result); // TODO check content validity
+            }catch(e){
+              alert("Could not parse '"+file.name+"', see console for details.");
+              console.error(e);
+              return;
+            }
+            
+            if (SAVEFILE.isValid(obj)){
+              STATE.uploadSavefile(new SAVEFILE(obj));
+            }
+            else{
+              alert("File '"+file.name+"' has an invalid format.");
+            }
+          };
+          
+          reader.readAsText(file, "UTF-8");
+        }
+
+        controller.ui.inputUpload.value = null;
+      });
+      
+      STATE.onStateChanged(() => {
+        if (controller){
+          controller.ui.btnDownload.disabled = controller.ui.btnReset.disabled = !STATE.hasSavedData();
+          controller.ui.btnToggleTracking.innerHTML = STATE.isTracking() ? "Pause Tracking" : "Start Tracking";
+        }
+      }, true);
+    },
+    
+    hideController: function(){
+      if (controller){
+        DOM.removeElement(controller.ele);
+        DOM.removeElement(controller.styles);
+        controller = null;
+      }
+    }
+  };
+  
+  return root;
+})();