diff --git a/Makefile b/Makefile
index 866ee2f34..7688025a5 100644
--- a/Makefile
+++ b/Makefile
@@ -215,6 +215,7 @@ EXTENSION_WEB_FILES = \
 	web/viewer.js \
 	web/viewer.html \
 	web/viewer-production.html \
+	web/debugger.js \
 	$(NULL)
 
 FIREFOX_BUILD_DIR := $(BUILD_DIR)/firefox
diff --git a/src/canvas.js b/src/canvas.js
index ec832d213..073c3d2d6 100644
--- a/src/canvas.js
+++ b/src/canvas.js
@@ -269,7 +269,8 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
     },
 
     executeIRQueue: function canvasGraphicsExecuteIRQueue(codeIR,
-                                  executionStartIdx, continueCallback) {
+                                  executionStartIdx, continueCallback,
+                                  stepper) {
       var argsArray = codeIR.argsArray;
       var fnArray = codeIR.fnArray;
       var i = executionStartIdx || 0;
@@ -288,6 +289,11 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
       var slowCommands = this.slowCommands;
 
       while (true) {
+        if (stepper && i === stepper.nextBreakPoint) {
+          stepper.breakIt(i, continueCallback);
+          return i;
+        }
+
         fnName = fnArray[i];
 
         if (fnName !== 'dependency') {
diff --git a/src/core.js b/src/core.js
index 137452257..86e3eeb5f 100644
--- a/src/core.js
+++ b/src/core.js
@@ -57,6 +57,7 @@ function getPdf(arg, callback) {
   xhr.send(null);
 }
 globalScope.PDFJS.getPdf = getPdf;
+globalScope.PDFJS.pdfBug = false;
 
 var Page = (function PageClosure() {
   function Page(xref, pageNumber, pageDict, ref) {
@@ -254,10 +255,16 @@ var Page = (function PageClosure() {
       var startIdx = 0;
       var length = this.IRQueue.fnArray.length;
       var IRQueue = this.IRQueue;
+      var stepper = null;
+      if (PDFJS.pdfBug && StepperManager.enabled) {
+        stepper = StepperManager.create(this.pageNumber);
+        stepper.init(IRQueue);
+        stepper.nextBreakPoint = stepper.getNextBreakPoint();
+      }
 
       var self = this;
       function next() {
-        startIdx = gfx.executeIRQueue(IRQueue, startIdx, next);
+        startIdx = gfx.executeIRQueue(IRQueue, startIdx, next, stepper);
         if (startIdx == length) {
           self.stats.render = Date.now();
           gfx.endDrawing();
diff --git a/src/fonts.js b/src/fonts.js
index 919441b58..0f9b6f9d0 100644
--- a/src/fonts.js
+++ b/src/fonts.js
@@ -2225,6 +2225,9 @@ var Font = (function FontClosure() {
       var styleSheet = styleElement.sheet;
       styleSheet.insertRule(rule, styleSheet.cssRules.length);
 
+      if (PDFJS.pdfBug && FontInspector.enabled)
+        FontInspector.fontAdded(this, url);
+
       return rule;
     },
 
diff --git a/web/debugger.js b/web/debugger.js
new file mode 100644
index 000000000..c759a5ee8
--- /dev/null
+++ b/web/debugger.js
@@ -0,0 +1,396 @@
+/* -*- Mode: Java; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set shiftwidth=2 tabstop=2 autoindent cindent expandtab: */
+
+'use strict';
+
+var FontInspector = (function FontInspectorClosure() {
+  var fonts;
+  var panelWidth = 300;
+  var active = false;
+  var fontAttribute = 'data-font-name';
+  function removeSelection() {
+    var divs = document.querySelectorAll('div[' + fontAttribute + ']');
+    for (var i = 0, ii = divs.length; i < ii; ++i) {
+      var div = divs[i];
+      div.className = '';
+    }
+  }
+  function resetSelection() {
+    var divs = document.querySelectorAll('div[' + fontAttribute + ']');
+    for (var i = 0, ii = divs.length; i < ii; ++i) {
+      var div = divs[i];
+      div.className = 'debuggerHideText';
+    }
+  }
+  function selectFont(fontName, show) {
+    var divs = document.querySelectorAll('div[' + fontAttribute + '=' +
+                                         fontName + ']');
+    for (var i = 0, ii = divs.length; i < ii; ++i) {
+      var div = divs[i];
+      div.className = show ? 'debuggerShowText' : 'debuggerHideText';
+    }
+  }
+  function textLayerClick(e) {
+    if (!e.target.dataset.fontName || e.target.tagName != 'DIV')
+      return;
+    var fontName = e.target.dataset.fontName;
+    var selects = document.getElementsByTagName('input');
+    for (var i = 0; i < selects.length; ++i) {
+      var select = selects[i];
+      if (select.dataset.fontName != fontName) continue;
+      select.checked = !select.checked;
+      selectFont(fontName, select.checked);
+      select.scrollIntoView();
+    }
+  }
+  return {
+    // Poperties/functions needed by PDFBug.
+    id: 'FontInspector',
+    name: 'Font Inspector',
+    panel: null,
+    manager: null,
+    init: function init() {
+      var panel = this.panel;
+      panel.setAttribute('style', 'padding: 5px;');
+      var tmp = document.createElement('button');
+      tmp.addEventListener('click', resetSelection);
+      tmp.textContent = 'Refresh';
+      panel.appendChild(tmp);
+
+      fonts = document.createElement('div');
+      panel.appendChild(fonts);
+    },
+    enabled: false,
+    get active() {
+      return active;
+    },
+    set active(value) {
+      active = value;
+      if (active) {
+        document.body.addEventListener('click', textLayerClick, true);
+        resetSelection();
+      } else {
+        document.body.removeEventListener('click', textLayerClick, true);
+        removeSelection();
+      }
+    },
+    // FontInspector specific functions.
+    fontAdded: function fontAdded(fontObj, url) {
+      function properties(obj, list) {
+        var moreInfo = document.createElement('table');
+        for (var i = 0; i < list.length; i++) {
+          var tr = document.createElement('tr');
+          var td1 = document.createElement('td');
+          td1.textContent = list[i];
+          tr.appendChild(td1);
+          var td2 = document.createElement('td');
+          td2.textContent = obj[list[i]].toString();
+          tr.appendChild(td2);
+          moreInfo.appendChild(tr);
+        }
+        return moreInfo;
+      }
+      var moreInfo = properties(fontObj, ['name', 'type']);
+      var m = /url\(['"]?([^\)"']+)/.exec(url);
+      var fontName = fontObj.loadedName;
+      var font = document.createElement('div');
+      var name = document.createElement('span');
+      name.textContent = fontName;
+      var download = document.createElement('a');
+      download.href = m[1];
+      download.textContent = 'Download';
+      var logIt = document.createElement('a');
+      logIt.href = '';
+      logIt.textContent = 'Log';
+      logIt.addEventListener('click', function(event) {
+        event.preventDefault();
+        console.log(fontObj);
+      });
+      var select = document.createElement('input');
+      select.setAttribute('type', 'checkbox');
+      select.dataset.fontName = fontName;
+      select.addEventListener('click', (function(select, fontName) {
+        return (function() {
+           selectFont(fontName, select.checked);
+        });
+      })(select, fontName));
+      font.appendChild(select);
+      font.appendChild(name);
+      font.appendChild(document.createTextNode(' '));
+      font.appendChild(download);
+      font.appendChild(document.createTextNode(' '));
+      font.appendChild(logIt);
+      font.appendChild(moreInfo);
+      fonts.appendChild(font);
+      // Somewhat of a hack, should probably add a hook for when the text layer
+      // is done rendering.
+      setTimeout(function() {
+        if (this.active)
+          resetSelection();
+      }.bind(this), 2000);
+    }
+  };
+})();
+
+// Manages all the page steppers.
+var StepperManager = (function StepperManagerClosure() {
+  var steppers = [];
+  var stepperDiv = null;
+  var stepperControls = null;
+  var stepperChooser = null;
+  var breakPoints = {};
+  return {
+    // Poperties/functions needed by PDFBug.
+    id: 'Stepper',
+    name: 'Stepper',
+    panel: null,
+    manager: null,
+    init: function init() {
+      var self = this;
+      this.panel.setAttribute('style', 'padding: 5px;');
+      stepperControls = document.createElement('div');
+      stepperChooser = document.createElement('select');
+      stepperChooser.addEventListener('change', function(event) {
+        self.selectStepper(this.value);
+      });
+      stepperControls.appendChild(stepperChooser);
+      stepperDiv = document.createElement('div');
+      this.panel.appendChild(stepperControls);
+      this.panel.appendChild(stepperDiv);
+      if (sessionStorage.getItem('pdfjsBreakPoints'))
+        breakPoints = JSON.parse(sessionStorage.getItem('pdfjsBreakPoints'));
+    },
+    enabled: false,
+    active: false,
+    // Stepper specific functions.
+    create: function create(pageNumber) {
+      var debug = document.createElement('div');
+      debug.id = 'stepper' + pageNumber;
+      debug.setAttribute('hidden', true);
+      debug.className = 'stepper';
+      stepperDiv.appendChild(debug);
+      var b = document.createElement('option');
+      b.textContent = 'Page ' + (pageNumber + 1);
+      b.value = pageNumber;
+      stepperChooser.appendChild(b);
+      var initBreakPoints = breakPoints[pageNumber] || [];
+      var stepper = new Stepper(debug, pageNumber, initBreakPoints);
+      steppers.push(stepper);
+      if (steppers.length === 1)
+        this.selectStepper(pageNumber, false);
+      return stepper;
+    },
+    selectStepper: function selectStepper(pageNumber, selectPanel) {
+      if (selectPanel)
+        this.manager.selectPanel(1);
+      for (var i = 0; i < steppers.length; ++i) {
+        var stepper = steppers[i];
+        if (stepper.pageNumber == pageNumber)
+          stepper.panel.removeAttribute('hidden');
+        else
+          stepper.panel.setAttribute('hidden', true);
+      }
+    },
+    saveBreakPoints: function saveBreakPoints(pageNumber, bps) {
+      breakPoints[pageNumber] = bps;
+      sessionStorage.setItem('pdfjsBreakPoints', JSON.stringify(breakPoints));
+    }
+  };
+})();
+
+// The stepper for each page's IRQueue.
+var Stepper = (function StepperClosure() {
+  function Stepper(panel, pageNumber, initialBreakPoints) {
+    this.panel = panel;
+    this.len;
+    this.breakPoint = 0;
+    this.nextBreakPoint = null;
+    this.pageNumber = pageNumber;
+    this.breakPoints = initialBreakPoints;
+    this.currentIdx = -1;
+  }
+  Stepper.prototype = {
+    init: function init(IRQueue) {
+      // Shorter way to create element and optionally set textContent.
+      function c(tag, textContent) {
+        var d = document.createElement(tag);
+        if (textContent)
+          d.textContent = textContent;
+        return d;
+      }
+      var panel = this.panel;
+      this.len = IRQueue.fnArray.length;
+      var content = c('div', 'c=continue, s=step');
+      var table = c('table');
+      content.appendChild(table);
+      table.cellSpacing = 0;
+      var headerRow = c('tr');
+      table.appendChild(headerRow);
+      headerRow.appendChild(c('th', 'Break'));
+      headerRow.appendChild(c('th', 'Idx'));
+      headerRow.appendChild(c('th', 'fn'));
+      headerRow.appendChild(c('th', 'args'));
+
+      for (var i = 0; i < IRQueue.fnArray.length; i++) {
+        var line = c('tr');
+        line.className = 'line';
+        line.id = 'idx' + i;
+        table.appendChild(line);
+        var checked = this.breakPoints.indexOf(i) != -1;
+        var args = IRQueue.argsArray[i] ? IRQueue.argsArray[i] : [];
+
+        var breakCell = c('td');
+        var cbox = c('input');
+        cbox.type = 'checkbox';
+        cbox.className = 'points';
+        cbox.checked = checked;
+        var self = this;
+        cbox.onclick = (function(x) {
+          return function() {
+            if (this.checked)
+              self.breakPoints.push(x);
+            else
+              self.breakPoints.splice(self.breakPoints.indexOf(x), 1);
+            StepperManager.saveBreakPoints(self.pageNumber, self.breakPoints);
+          }
+        })(i);
+
+        breakCell.appendChild(cbox);
+        line.appendChild(breakCell);
+        line.appendChild(c('td', i.toString()));
+        line.appendChild(c('td', IRQueue.fnArray[i]));
+        line.appendChild(c('td', args.join(', ')));
+      }
+      panel.appendChild(content);
+      var self = this;
+    },
+    getNextBreakPoint: function getNextBreakPoint() {
+      this.breakPoints.sort(function(a, b) { return a - b; });
+      for (var i = 0; i < this.breakPoints.length; i++) {
+        if (this.breakPoints[i] > this.currentIdx)
+          return this.breakPoints[i];
+      }
+      return null;
+    },
+    breakIt: function breakIt(idx, callback) {
+      StepperManager.selectStepper(this.pageNumber, true);
+      var self = this;
+      var dom = document;
+      self.currentIdx = idx;
+      var listener = function(e) {
+        switch (e.keyCode) {
+          case 83: // step
+            dom.removeEventListener('keydown', listener, false);
+            self.nextBreakPoint = self.currentIdx + 1;
+            self.goTo(-1);
+            callback();
+            break;
+          case 67: // continue
+            dom.removeEventListener('keydown', listener, false);
+            var breakPoint = self.getNextBreakPoint();
+            self.nextBreakPoint = breakPoint;
+            self.goTo(-1);
+            callback();
+            break;
+        }
+      }
+      dom.addEventListener('keydown', listener, false);
+      self.goTo(idx);
+    },
+    goTo: function goTo(idx) {
+      var allRows = this.panel.getElementsByClassName('line');
+      for (var x = 0; x < allRows.length; x++) {
+        allRows[x].style.backgroundColor = null;
+      }
+      var row = document.getElementById('idx' + idx);
+      if (row) {
+        row.style.backgroundColor = 'rgb(251,250,207)';
+        row.scrollIntoView();
+      }
+    }
+  };
+  return Stepper;
+})();
+
+// Manages all the debugging tools.
+var PDFBug = (function PDFBugClosure() {
+  var panelWidth = 300;
+  var buttons = [];
+  var activePanel = null;
+
+  return {
+    tools: [
+      FontInspector,
+      StepperManager
+    ],
+    init: function init() {
+      /*
+       * Basic Layout:
+       * PDFBug
+       *  Controls
+       *  Panels
+       *    Panel
+       *    Panel
+       *    ...
+       */
+      var ui = document.createElement('div');
+      ui.id = 'PDFBug';
+
+      var controls = document.createElement('div');
+      controls.setAttribute('class', 'controls');
+      ui.appendChild(controls);
+
+      var panels = document.createElement('div');
+      panels.setAttribute('class', 'panels');
+      ui.appendChild(panels);
+
+      document.body.appendChild(ui);
+      document.body.style.paddingRight = panelWidth + 'px';
+
+      // Initialize all the debugging tools.
+      var tools = this.tools;
+      for (var i = 0; i < tools.length; ++i) {
+        var tool = tools[i];
+        var panel = document.createElement('div');
+        var panelButton = document.createElement('button');
+        panelButton.textContent = tool.name;
+        var self = this;
+        panelButton.addEventListener('click', (function(selected) {
+          return function(event) {
+            event.preventDefault();
+            self.selectPanel(selected);
+          };
+        })(i));
+        controls.appendChild(panelButton);
+        panels.appendChild(panel);
+        tool.panel = panel;
+        tool.manager = this;
+        if (tool.enabled)
+          tool.init();
+        else
+          panel.textContent = tool.name + ' is disabled. To enable add ' +
+                              ' "' + tool.id + '" to the pdfBug parameter ' +
+                              'and refresh (seperate multiple by commas).';
+        buttons.push(panelButton);
+      }
+      this.selectPanel(0);
+    },
+    selectPanel: function selectPanel(index) {
+      if (index === activePanel)
+        return;
+      activePanel = index;
+      var tools = this.tools;
+      for (var j = 0; j < tools.length; ++j) {
+        if (j == index) {
+          buttons[j].setAttribute('class', 'active');
+          tools[j].active = true;
+          tools[j].panel.removeAttribute('hidden');
+        } else {
+          buttons[j].setAttribute('class', '');
+          tools[j].active = false;
+          tools[j].panel.setAttribute('hidden', 'true');
+        }
+      }
+    }
+  };
+})();
diff --git a/web/viewer.css b/web/viewer.css
index ddf3b0985..befaf7aa6 100644
--- a/web/viewer.css
+++ b/web/viewer.css
@@ -405,3 +405,40 @@ canvas {
   margin: 100px 0;
   text-align: center;
 }
+
+#PDFBug {
+  font-size: 10px;
+  position: fixed;
+  top: 35px;
+  bottom: 5px;
+  right: 2px;
+  width: 300px;
+  background: white;
+  border: 1px solid #666;
+  padding: 0;
+}
+#PDFBug .controls {
+  border-bottom: 1px solid #666;
+  padding: 3px;
+  background: -moz-linear-gradient(center bottom, #eee 0%, #fff 100%);
+}
+#PDFBug .panels {
+  overflow: auto;
+  position: absolute;
+  top: 27px;
+  left: 0;
+  right: 0;
+  bottom: 0;
+}
+#PDFBug button.active {
+  font-weight: bold;
+}
+.debuggerShowText {
+  background: yellow;
+  color: blue;
+  opacity: 0.3;
+}
+.debuggerHideText:hover {
+  background: yellow;
+  opacity: 0.3;
+}
diff --git a/web/viewer.html b/web/viewer.html
index e136260e8..ee992a2c5 100644
--- a/web/viewer.html
+++ b/web/viewer.html
@@ -30,8 +30,10 @@
         <script type="text/javascript" src="../external/jpgjs/jpg.js"></script>  <!-- PDFJSSCRIPT_REMOVE_CORE -->
         <script type="text/javascript" src="../src/jpx.js"></script>  <!-- PDFJSSCRIPT_REMOVE_CORE -->
         <script type="text/javascript">PDFJS.workerSrc = '../src/worker_loader.js';</script> <!-- PDFJSSCRIPT_REMOVE_CORE -->
+        <script type="text/javascript" src="debugger.js"></script>
         <script type="text/javascript" src="viewer.js"></script>
 
+
     </head>
 
   <body>
diff --git a/web/viewer.js b/web/viewer.js
index dd4ddaedc..5e94c2241 100644
--- a/web/viewer.js
+++ b/web/viewer.js
@@ -504,13 +504,7 @@ var PDFView = {
       return;
 
     if (hash.indexOf('=') >= 0) {
-      // parsing query string
-      var paramsPairs = hash.split('&');
-      var params = {};
-      for (var i = 0; i < paramsPairs.length; ++i) {
-        var paramPair = paramsPairs[i].split('=');
-        params[paramPair[0]] = paramPair[1];
-      }
+      var params = PDFView.parseQueryString(hash);
       // borrowing syntax from "Parameters for Opening PDF Files"
       if ('nameddest' in params) {
         PDFView.navigateTo(params.nameddest);
@@ -622,6 +616,16 @@ var PDFView = {
     }
 
     return visibleThumbs;
+  },
+
+  // Helper function to parse query string (e.g. ?param1=value&parm2=...).
+  parseQueryString: function pdfViewParseQueryString(query) {
+    var params = query.split('&');
+    for (var i = 0; i < params.length; i++) {
+      var param = params[i].split('=');
+      params[unescape(param[0])] = unescape(param[1]);
+    }
+    return params;
   }
 };
 
@@ -1083,6 +1087,7 @@ var TextLayerBuilder = function textLayerBuilder(textLayerDiv) {
     // vScale and hScale already contain the scaling to pixel units
     var fontHeight = fontSize * text.geom.vScale;
     textDiv.dataset.canvasWidth = text.canvasWidth * text.geom.hScale;
+    textDiv.dataset.fontName = fontName;
 
     textDiv.style.fontSize = fontHeight + 'px';
     textDiv.style.left = text.geom.x + 'px';
@@ -1094,16 +1099,11 @@ var TextLayerBuilder = function textLayerBuilder(textLayerDiv) {
 };
 
 window.addEventListener('load', function webViewerLoad(evt) {
-  var params = document.location.search.substring(1).split('&');
-  for (var i = 0; i < params.length; i++) {
-    var param = params[i].split('=');
-    params[unescape(param[0])] = unescape(param[1]);
-  }
+  var params = PDFView.parseQueryString(document.location.search.substring(1));
 
-  var scale = ('scale' in params) ? params.scale : 0;
   var file = PDFJS.isFirefoxExtension ?
               window.location.toString() : params.file || kDefaultURL;
-  PDFView.open(file, parseFloat(scale));
+  PDFView.open(file, 0);
 
   if (PDFJS.isFirefoxExtension || !window.File || !window.FileReader ||
       !window.FileList || !window.Blob) {
@@ -1114,11 +1114,32 @@ window.addEventListener('load', function webViewerLoad(evt) {
     document.getElementById('fileInput').value = null;
   }
 
-  if ('disableWorker' in params)
-    PDFJS.disableWorker = (params['disableWorker'] === 'true');
+  // Special debugging flags in the hash section of the URL.
+  var hash = document.location.hash.substring(1);
+  var hashParams = PDFView.parseQueryString(hash);
 
-  if ('disableTextLayer' in params)
-    PDFJS.disableTextLayer = (params['disableTextLayer'] === 'true');
+  if ('disableWorker' in hashParams)
+    PDFJS.disableWorker = (hashParams['disableWorker'] === 'true');
+
+  if ('disableTextLayer' in hashParams)
+    PDFJS.disableTextLayer = (hashParams['disableTextLayer'] === 'true');
+
+  if ('pdfBug' in hashParams) {
+    PDFJS.pdfBug = true;
+    var pdfBug = hashParams['pdfBug'];
+    var all = false, enabled = [];
+    if (pdfBug === 'all')
+      all = true;
+    else
+      enabled = pdfBug.split(',');
+    var debugTools = PDFBug.tools;
+    for (var i = 0; i < debugTools.length; ++i) {
+      var tool = debugTools[i];
+      if (all || enabled.indexOf(tool.id) !== -1)
+        tool.enabled = true;
+    }
+    PDFBug.init();
+  }
 
   var sidebarScrollView = document.getElementById('sidebarScrollView');
   sidebarScrollView.addEventListener('scroll', updateThumbViewArea, true);