diff --git a/gen/mixins/__init__.py b/gen/mixins/__init__.py
index 1f20560..7061dab 100644
--- a/gen/mixins/__init__.py
+++ b/gen/mixins/__init__.py
@@ -98,6 +98,8 @@ class BaseMixin:
         if not created:
             from DateTime import DateTime
             obj.modified = DateTime()
+        # Unlock the currently saved page on the object
+        if rq: self.removeLock(rq['page'])
         obj.reindex()
         return obj, msg
 
@@ -188,6 +190,53 @@ class BaseMixin:
         obj = createObject(tool.getPath('/temp_folder'), id, className, appName)
         return self.goto(obj.getUrl(**urlParams))
 
+    def setLock(self, user, page):
+        '''A p_user edits a given p_page on this object: we will set a lock, to
+           prevent other users to edit this page at the same time.'''
+        if not hasattr(self.aq_base, 'locks'):
+            # Create the persistent mapping that will store the lock
+            # ~{s_page: s_userId}~
+            from persistent.mapping import PersistentMapping
+            self.locks = PersistentMapping()
+        # Raise an error is the page is already locked by someone else. If the
+        # page is already locked by the same user, we don't mind: he could have
+        # used back/forward buttons of its browser...
+        userId = user.getId()
+        if (page in self.locks) and (userId != self.locks[page]):
+            from AccessControl import Unauthorized
+            raise Unauthorized('This page is locked.')
+        # Set the lock
+        self.locks[page] = userId
+
+    def isLocked(self, user, page):
+        '''Is this page locked? If the page is locked by the same user, we don't
+           mind and consider the page as unlocked. If the page is locked, this
+           method returns the id of the user that has locked the page.'''
+        if hasattr(self.aq_base, 'locks') and (page in self.locks):
+            if (user.getId() != self.locks[page]): return self.locks[page]
+
+    def removeLock(self, page):
+        '''Removes the lock on the current page. This happens after the page has
+           been saved: the lock must be released.'''
+        if page not in self.locks: return
+        # Raise an error if the user that saves changes is not the one that
+        # has locked the page.
+        userId = self.getUser().getId()
+        if self.locks[page] != userId:
+            from AccessControl import Unauthorized
+            raise Unauthorized('This page was locked by someone else.')
+        # Remove the lock
+        del self.locks[page]
+
+    def removeMyLock(self, user, page):
+        '''If p_user has set a lock on p_page, this method removes it. This
+           method is called when the user that locked a page consults
+           view.pt for this page. In this case, we consider that the user has
+           left the edit page in an unexpected way and we remove the lock.'''
+        if hasattr(self.aq_base, 'locks') and (page in self.locks) and \
+           (user.getId() == self.locks[page]):
+            del self.locks[page]
+
     def onCreateWithoutForm(self):
         '''This method is called when a user wants to create a object from a
            reference field, automatically (without displaying a form).'''
@@ -263,6 +312,7 @@ class BaseMixin:
                 else:
                    urlBack = self.getUrl()
             self.say(self.translate('object_canceled'))
+            self.removeLock(rq['page'])
             return self.goto(urlBack)
 
         # Object for storing validation errors
diff --git a/gen/po.py b/gen/po.py
index 83b3308..fc21943 100644
--- a/gen/po.py
+++ b/gen/po.py
@@ -201,6 +201,7 @@ appyLabels = [
  ('changes_show', 'Show changes'),
  ('changes_hide', 'Hide changes'),
  ('anonymous', 'an anonymous user'),
+ ('page_locked', 'This page is locked by ${user}.'),
 ]
 
 # Some default values for labels whose ids are not fixed (so they can't be
diff --git a/gen/ui/edit.gif b/gen/ui/edit.gif
deleted file mode 100644
index 5fd9cfa..0000000
Binary files a/gen/ui/edit.gif and /dev/null differ
diff --git a/gen/ui/edit.png b/gen/ui/edit.png
new file mode 100644
index 0000000..f31f10d
Binary files /dev/null and b/gen/ui/edit.png differ
diff --git a/gen/ui/edit.pt b/gen/ui/edit.pt
index d4facda..c99ee50 100644
--- a/gen/ui/edit.pt
+++ b/gen/ui/edit.pt
@@ -8,6 +8,7 @@
                 phaseInfo      python: contextObj.getAppyPhases(currentOnly=True, layoutType=layoutType);
                 phase          phaseInfo/name;
                 page           request/page|python:contextObj.getDefaultEditPage();
+                dummy          python: contextObj.setLock(user, page);
                 confirmMsg     request/confirmMsg | nothing;
                 groupedWidgets python: contextObj.getGroupedAppyTypes(layoutType, page, cssJs=cssJs);"
     tal:on-error="structure python: tool.manageError(error)">
diff --git a/gen/ui/locked.png b/gen/ui/locked.png
new file mode 100644
index 0000000..8b81e7f
Binary files /dev/null and b/gen/ui/locked.png differ
diff --git a/gen/ui/portlet.pt b/gen/ui/portlet.pt
index 88dbd1e..8e00b8d 100644
--- a/gen/ui/portlet.pt
+++ b/gen/ui/portlet.pt
@@ -115,7 +115,8 @@
 </tal:comment>
 <table metal:define-macro="phases" class="portletContent"
        tal:define="singlePhase python: phases and (len(phases) == 1);
-                   page python: req.get('page', 'main')">
+                   page python: req.get('page', 'main');
+                   mayEdit python: contextObj.allows('Modify portal content')">
   <tal:phase repeat="phase phases">
     <tal:comment replace="nothing">The box containing phase-related information</tal:comment>
     <tr tal:define="singlePage python: len(phase['pages']) == 1">
@@ -132,10 +133,19 @@
           <a tal:attributes="href python: contextObj.getUrl(page=aPage)"
              tal:content="structure python: _('%s_page_%s' % (contextObj.meta_type, aPage))">
           </a>
-          <a tal:condition="python: contextObj.allows('Modify portal content') and phase['pagesInfo'][aPage]['showOnEdit']"
-             tal:attributes="href python: contextObj.getUrl(mode='edit', page=aPage)">
-           <img title="Edit" tal:attributes="src string: $appUrl/ui/edit.gif"/>
-          </a>
+          <tal:icons define="locked python: contextObj.isLocked(user, aPage);
+                             editable python: mayEdit and phase['pagesInfo'][aPage]['showOnEdit']">
+           <a tal:condition="python: editable and not locked"
+              tal:attributes="href python: contextObj.getUrl(mode='edit', page=aPage)">
+            <img tal:attributes="src string: $appUrl/ui/edit.png;
+                                 title python: _('object_edit')"/>
+           </a>
+           <a tal:condition="python: editable and locked">
+            <img style="cursor: help"
+                 tal:attributes="src string: $appUrl/ui/locked.png;
+                                 title python: _('page_locked', mapping={'user':tool.getUserName(locked)})"/>
+           </a>
+          </tal:icons>
         </div>
         <tal:comment replace="nothing">Next lines: links</tal:comment>
         <tal:links define="links python: phase['pagesInfo'][aPage].get('links')" tal:condition="links">
diff --git a/gen/ui/result.pt b/gen/ui/result.pt
index 496fceb..278db68 100644
--- a/gen/ui/result.pt
+++ b/gen/ui/result.pt
@@ -95,7 +95,7 @@
          <a tal:define="navInfo python:'search.%s.%s.%d.%d' % (className, searchName, repeat['obj'].number()+startNumber, totalNumber);"
             tal:attributes="href python: obj.getUrl(mode='edit', page=obj.getDefaultEditPage(), nav=navInfo)"
             tal:condition="obj/mayEdit">
-          <img tal:attributes="src string: $appUrl/ui/edit.gif;
+          <img tal:attributes="src string: $appUrl/ui/edit.png;
                                title python: _('object_edit')"/></a><img
             tal:condition="obj/mayDelete" style="cursor:pointer"
             tal:attributes="src string: $appUrl/ui/delete.png;
diff --git a/gen/ui/template.pt b/gen/ui/template.pt
index 0abb5b3..9622d55 100644
--- a/gen/ui/template.pt
+++ b/gen/ui/template.pt
@@ -175,7 +175,7 @@
           <span tal:content="python: userInfo[0]"></span>
           <a tal:condition="python: userInfo[1]"
              tal:attributes="href python: userInfo[1]">
-            <img tal:attributes="src string: $appUrl/ui/edit.gif"/>
+            <img tal:attributes="src string: $appUrl/ui/edit.png"/>
           </a>
         </td>
        </tr>
diff --git a/gen/ui/view.pt b/gen/ui/view.pt
index 0e3d108..d0e5695 100644
--- a/gen/ui/view.pt
+++ b/gen/ui/view.pt
@@ -8,6 +8,7 @@
                 phase          phaseInfo/name;
                 cssJs          python: {};
                 page           req/page|python:contextObj.getDefaultViewPage();
+                dummy          python: contextObj.removeMyLock(user, page);
                 groupedWidgets python: contextObj.getGroupedAppyTypes(layoutType, page, cssJs=cssJs);"
     tal:on-error="structure python: tool.manageError(error)">
 
diff --git a/gen/ui/widgets/ref.pt b/gen/ui/widgets/ref.pt
index 739dd54..e8434e8 100644
--- a/gen/ui/widgets/ref.pt
+++ b/gen/ui/widgets/ref.pt
@@ -27,7 +27,7 @@
     <td tal:condition="python: not appyType['noForm'] and obj.mayEdit() and appyType['delete']">
        <a tal:define="navInfo python:'ref.%s.%s:%s.%d.%d' % (contextObj.UID(), fieldName, appyType['pageName'], repeat['obj'].number()+startNumber, totalNumber);"
           tal:attributes="href python: obj.getUrl(mode='edit', page='main', nav=navInfo)">
-        <img tal:attributes="src string: $appUrl/ui/edit.gif;
+        <img tal:attributes="src string: $appUrl/ui/edit.png;
                              title python: _('object_edit')"/>
        </a>
     </td>