diff --git a/src/TDB2.cpp b/src/TDB2.cpp
index 6f3a6f00e..6a3f4570f 100644
--- a/src/TDB2.cpp
+++ b/src/TDB2.cpp
@@ -567,6 +567,345 @@ void TDB2::synch ()
   context.timer_synch.stop ();
 }
 
+////////////////////////////////////////////////////////////////////////////////
+void TDB2::revert ()
+{
+/*
+  Directory location (context.config.get ("data.location"));
+
+  std::string undoFile      = location._data + "/undo.data";
+  std::string pendingFile   = location._data + "/pending.data";
+  std::string completedFile = location._data + "/completed.data";
+
+  // load undo.data
+  std::vector <std::string> u;
+  File::read (undoFile, u);
+
+  if (u.size () < 3)
+    throw std::string ("There are no recorded transactions to undo.");
+
+  // pop last tx
+  u.pop_back (); // separator.
+
+  std::string current = u.back ().substr (4);
+  u.pop_back ();
+
+  std::string prior;
+  std::string when;
+  if (u.back ().substr (0, 5) == "time ")
+  {
+    when = u.back ().substr (5);
+    u.pop_back ();
+    prior = "";
+  }
+  else
+  {
+    prior = u.back ().substr (4);
+    u.pop_back ();
+    when = u.back ().substr (5);
+    u.pop_back ();
+  }
+
+  Date lastChange (atoi (when.c_str ()));
+
+  // Set the colors.
+  Color color_red   (context.color () ? context.config.get ("color.undo.before") : "");
+  Color color_green (context.color () ? context.config.get ("color.undo.after") : "");
+
+  if (context.config.get ("undo.style") == "side")
+  {
+    std::cout << "\n"
+              << "The last modification was made "
+              << lastChange.toString ()
+              << "\n";
+
+    // Attributes are all there is, so figure the different attribute names
+    // between before and after.
+    ViewText view;
+    view.width (context.getWidth ());
+    view.intraPadding (2);
+    view.add (Column::factory ("string", ""));
+    view.add (Column::factory ("string", "Prior Values"));
+    view.add (Column::factory ("string", "Current Values"));
+
+    Task after (current);
+
+    if (prior != "")
+    {
+      Task before (prior);
+
+      std::vector <std::string> beforeAtts;
+      foreach (att, before)
+        beforeAtts.push_back (att->first);
+
+      std::vector <std::string> afterAtts;
+      foreach (att, after)
+        afterAtts.push_back (att->first);
+
+      std::vector <std::string> beforeOnly;
+      std::vector <std::string> afterOnly;
+      listDiff (beforeAtts, afterAtts, beforeOnly, afterOnly);
+
+      int row;
+      foreach (name, beforeOnly)
+      {
+        row = view.addRow ();
+        view.set (row, 0, *name);
+        view.set (row, 1, renderAttribute (*name, before.get (*name)), color_red);
+      }
+
+      foreach (name, before)
+      {
+        std::string priorValue   = before.get (name->first);
+        std::string currentValue = after.get  (name->first);
+
+        if (currentValue != "")
+        {
+          row = view.addRow ();
+          view.set (row, 0, name->first);
+          view.set (row, 1, renderAttribute (name->first, priorValue),
+                    (priorValue != currentValue ? color_red : Color ()));
+          view.set (row, 2, renderAttribute (name->first, currentValue),
+                    (priorValue != currentValue ? color_green : Color ()));
+        }
+      }
+
+      foreach (name, afterOnly)
+      {
+        row = view.addRow ();
+        view.set (row, 0, *name);
+        view.set (row, 2, renderAttribute (*name, after.get (*name)), color_green);
+      }
+    }
+    else
+    {
+      int row;
+      foreach (name, after)
+      {
+        row = view.addRow ();
+        view.set (row, 0, name->first);
+        view.set (row, 2, renderAttribute (name->first, after.get (name->first)), color_green);
+      }
+    }
+
+    std::cout << "\n"
+              << view.render ()
+              << "\n";
+  }
+
+  // This style looks like this:
+  //  --- before    2009-07-04 00:00:25.000000000 +0200
+  //  +++ after    2009-07-04 00:00:45.000000000 +0200
+  //
+  // - name: old           // att deleted
+  // + name:
+  //
+  // - name: old           // att changed
+  // + name: new
+  //
+  // - name:
+  // + name: new           // att added
+  //
+  else if (context.config.get ("undo.style") == "diff")
+  {
+    // Create reference tasks.
+    Task before;
+    if (prior != "")
+      before.parse (prior);
+
+    Task after (current);
+
+    // Generate table header.
+    ViewText view;
+    view.width (context.getWidth ());
+    view.intraPadding (2);
+    view.add (Column::factory ("string", ""));
+    view.add (Column::factory ("string", ""));
+
+    int row = view.addRow ();
+    view.set (row, 0, "--- previous state", color_red);
+    view.set (row, 1, "Undo will restore this state", color_red);
+
+    row = view.addRow ();
+    view.set (row, 0, "+++ current state ", color_green);  // Note trailing space.
+    view.set (row, 1, "Change made " + lastChange.toString (context.config.get ("dateformat")), color_green);
+
+    view.addRow ();
+
+    // Add rows to table showing diffs.
+    std::vector <std::string> all = context.getColumns ();
+
+    // Now factor in the annotation attributes.
+    Task::iterator it;
+    for (it = before.begin (); it != before.end (); ++it)
+      if (it->first.substr (0, 11) == "annotation_")
+        all.push_back (it->first);
+
+    for (it = after.begin (); it != after.end (); ++it)
+      if (it->first.substr (0, 11) == "annotation_")
+        all.push_back (it->first);
+
+    // Now render all the attributes.
+    std::sort (all.begin (), all.end ());
+
+    std::string before_att;
+    std::string after_att;
+    std::string last_att;
+    foreach (a, all)
+    {
+      if (*a != last_att)  // Skip duplicates.
+      {
+        last_att = *a;
+
+        before_att = before.get (*a);
+        after_att  = after.get (*a);
+
+        // Don't report different uuid.
+        // Show nothing if values are the unchanged.
+        if (*a == "uuid" ||
+            before_att == after_att)
+        {
+          // Show nothing - no point displaying that which did not change.
+
+          // row = view.addRow ();
+          // view.set (row, 0, *a + ":");
+          // view.set (row, 1, before_att);
+        }
+
+        // Attribute deleted.
+        else if (before_att != "" && after_att == "")
+        {
+          row = view.addRow ();
+          view.set (row, 0, "-" + *a + ":", color_red);
+          view.set (row, 1, before_att, color_red);
+
+          row = view.addRow ();
+          view.set (row, 0, "+" + *a + ":", color_green);
+        }
+
+        // Attribute added.
+        else if (before_att == "" && after_att != "")
+        {
+          row = view.addRow ();
+          view.set (row, 0, "-" + *a + ":", color_red);
+
+          row = view.addRow ();
+          view.set (row, 0, "+" + *a + ":", color_green);
+          view.set (row, 1, after_att, color_green);
+        }
+
+        // Attribute changed.
+        else
+        {
+          row = view.addRow ();
+          view.set (row, 0, "-" + *a + ":", color_red);
+          view.set (row, 1, before_att, color_red);
+
+          row = view.addRow ();
+          view.set (row, 0, "+" + *a + ":", color_green);
+          view.set (row, 1, after_att, color_green);
+        }
+      }
+    }
+
+    std::cout << "\n"
+              << view.render ()
+              << "\n";
+  }
+
+  // Output displayed, now confirm.
+  if (context.config.getBoolean ("confirmation") &&
+      !confirm ("The undo command is not reversible.  Are you sure you want to revert to the previous state?"))
+  {
+    std::cout << "No changes made.\n";
+    return;
+  }
+
+  // Extract identifying uuid.
+  std::string uuid;
+  std::string::size_type uuidAtt = current.find ("uuid:\"");
+  if (uuidAtt != std::string::npos)
+    uuid = current.substr (uuidAtt, 43); // 43 = uuid:"..."
+  else
+    throw std::string ("Cannot locate UUID in task to undo.");
+
+  // load pending.data
+  std::vector <std::string> p;
+  File::read (pendingFile, p);
+
+  // is 'current' in pending?
+  foreach (task, p)
+  {
+    if (task->find (uuid) != std::string::npos)
+    {
+      context.debug ("TDB::undo - task found in pending.data");
+
+      // Either revert if there was a prior state, or remove the task.
+      if (prior != "")
+      {
+        *task = prior;
+        std::cout << "Modified task reverted.\n";
+      }
+      else
+      {
+        p.erase (task);
+        std::cout << "Task removed.\n";
+      }
+
+      // Rewrite files.
+      File::write (pendingFile, p);
+      File::write (undoFile, u);
+      return;
+    }
+  }
+
+  // load completed.data
+  std::vector <std::string> c;
+  File::read (completedFile, c);
+
+  // is 'current' in completed?
+  foreach (task, c)
+  {
+    if (task->find (uuid) != std::string::npos)
+    {
+      context.debug ("TDB::undo - task found in completed.data");
+
+      // If task now belongs back in pending.data
+      if (prior.find ("status:\"pending\"")   != std::string::npos ||
+          prior.find ("status:\"waiting\"")   != std::string::npos ||
+          prior.find ("status:\"recurring\"") != std::string::npos)
+      {
+        c.erase (task);
+        p.push_back (prior);
+        File::write (completedFile, c);
+        File::write (pendingFile, p);
+        File::write (undoFile, u);
+        std::cout << "Modified task reverted.\n";
+        context.debug ("TDB::undo - task belongs in pending.data");
+      }
+      else
+      {
+        *task = prior;
+        File::write (completedFile, c);
+        File::write (undoFile, u);
+        std::cout << "Modified task reverted.\n";
+        context.debug ("TDB::undo - task belongs in completed.data");
+      }
+
+      std::cout << "Undo complete.\n";
+      return;
+    }
+  }
+
+  // Perhaps user hand-edited the data files?
+  // Perhaps the task was in completed.data, which was still in file format 3?
+  std::cout << "Task with UUID "
+            << uuid.substr (6, 36)
+            << " not found in data.\n"
+            << "No undo possible.\n";
+*/
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Scans the pending tasks for any that are completed or deleted, and if so,
 // moves them to the completed.data file.  Returns a count of tasks moved.
diff --git a/src/TDB2.h b/src/TDB2.h
index e63b1d21e..8cf3cb25e 100644
--- a/src/TDB2.h
+++ b/src/TDB2.h
@@ -97,6 +97,7 @@ public:
   void modify (const Task&);
   void commit ();
   void synch ();
+  void revert ();
   int  gc ();
   int  next_id ();
 
diff --git a/src/commands/CmdUndo.cpp b/src/commands/CmdUndo.cpp
index 08635615e..f7705a3c9 100644
--- a/src/commands/CmdUndo.cpp
+++ b/src/commands/CmdUndo.cpp
@@ -46,11 +46,12 @@ CmdUndo::CmdUndo ()
 ////////////////////////////////////////////////////////////////////////////////
 int CmdUndo::execute (std::string& output)
 {
-  // TODO Detect attemps to modify the task.
+  // Detect attemps to modify the task.
+  if (context.a3.extract_modifications ().size () > 0)
+    throw STRING_CMD_UNDO_MODS;
 
-  context.tdb.lock (context.config.getBoolean ("locking"));
-  context.tdb.undo ();
-  context.tdb.unlock ();
+  context.tdb2.revert ();
+  context.tdb2.commit ();
   return 0;
 }
 
diff --git a/src/en-US.h b/src/en-US.h
index 6276fa19e..37d7f0b34 100644
--- a/src/en-US.h
+++ b/src/en-US.h
@@ -203,6 +203,7 @@
 #define STRING_CMD_INFO_MODIFICATION "Modification"
 #define STRING_CMD_INFO_TOTAL_ACTIVE "Total active time"
 #define STRING_CMD_UNDO_USAGE        "Reverts the most recent change to a task."
+#define STRING_CMD_UNDO_MODS         "The undo command does not allow further task modification."
 #define STRING_CMD_STATS_USAGE       "Shows task database statistics."
 #define STRING_CMD_STATS_CATEGORY    "Category"
 #define STRING_CMD_STATS_DATA        "Data"