O'Reilly Network: Top Ten Gotchas in Upgrading to .NET Framework 1.1 [May. 20, 2003] "There are layers and layers in .NET. A new release involves changes to all of these layers. First there's the CLR, which actually manages the details of executing .NET code. On top of that is the Framework Class Library (FCL), Microsoft's own set of useful classes (which are themselves .NET code). Visual Studio .NET and other development environments layer their own shortcuts and code generators on top of the FCL, and your application software sits at the top of the stack.
Some of the most interesting changes from .NET 1.0 to .NET 1.1 are in the FCL. Microsoft has catalogued over 100 breaking changes as a result of the new version. While many of these changes are in specialized areas that most .NET developers will never visit, there are some major changes that you should be aware of, lest they bite you when you upgrade."