Problem

I set a breakpoint and my debugger is not stopping at the breakpoint.  Debug is configured in the Web.Config:

<compilation debug="true">
    <assemblies>

My assembly is compliled in Debug mode:

Solution

The last thing to check is of cource the Project Properties. 

Go to the tab “Web”, just below Signing.

You will see at the bottom a section Debuggers.

Activate the ASP.NET checkbox and your breakpoints will halt your running code again.

Microsoft support can be found here for some common problems: INFO Common Error When you Debug ASP.Net Applications in Visual Studio .NET and especially here: PRB: Visual Studio .NET Debugger Does Not Stop on Breakpoints When You Debug ASP.NET Pages

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