.NET Framework 4 Client Profile

I had a problem where I was referencing Microsoft.IdentityModel, and using this as a using statement such as

using Microsoft.IdentityModel.Protocols.WSTrust;

but got the complier error 'the type or namespace name ... could not be found'. This was completely driving me nuts - lost 0.5 days on this. When I include the reference initially, intellisense would pick it up, but on compiling it failed with that error, then intellisense stopped picking it up.

It turns out that I was using '.NET Framework 4 Client Profile' as the target framework, on changing to '.NET Framework 4', it fixed the problem.