>>>I have "moved" (copied) my VS 2005 project (.NET 2.0) to VS 2012. Now (in VS 2012) under .NET 4.0 the validation does not work. Strange is that I created a test aspx page with the same validation as in one of the application pages; and the validation works in the test page. But not in one of the pages of the application. So there must be something different.
>>>
>>>How do you troubleshoot the validation? Where in the debugger (if this is where I should look) I can see the validation?
>>>
>>>This is the segment of code of my validation:
>>>

>>>  <asp:textbox id="RequestorPhone" CssClass="FieldRW" MaxLength="14" Runat="server"></asp:textbox>
>>>			<asp:requiredfieldvalidator id="rfvRequestorPhone" runat="server" CssClass="FieldLabel" ControlToValidate="RequestorPhone"
>>>			ErrorMessage="RequiredFieldValidator" Enabled="True">  Please enter your telephone</asp:requiredfieldvalidator>  
>>>        <asp:button id="Submit" Runat="server" Text="Submit" ></asp:button>  
>>>

>>
>>Do you mean you don't see errors when you press Submit button? The above looks like server side validation, does the test page show errors right away (client side validation)?
>
>Yes, when I click on Submit the page, it is submitted and no error is shown. But I just added the controls (as shown above) as test to the application page with a test button. And when I click on Test Submit button, the error is shown and the page is not submitted. But when I click on the application "submit" button the page is submitted without a message. So on the same page I have an issue and don't have an issue. I will continue exploring.

May be that Submit button doesn't do anything? Did you trace its Click event?