Well after swearing at the screen and jumping up and down for the last hour I have figured out my problem: which was getting an error message when trying to add an ASP.NET custom control to the toolbox in Visual Studio .NET 2003. The message was something linke "Unable to load assembly <assmebly path>. Check that any required dependencies are installed". The .dll in question was a very simple ASP.NET control library containing 2 custom controls I wrote ages ago. After mucking around with and opening and closing files, re-compiling and citing various ancient religious chants I had an epiphany. The .dll I was trying to add was on our build server, which I hadn't set up a .NET security policy for! So I just added a new Code Group for the build server path with FullTrust rights and hey presto, the thing added! Hope this helps someone.