.NET Compatibility

What .NET version are completely compatible?

What I mean is, at our company we have at least one appplication
developed in .NET 1.0 that does not work when using .NET 1.1 runtime.
The problem apparently lies with changes to an xml class. So in my
opinion it's not completely safe to run an .NET 1.0 application
using .NET 1.1 runtime. How big a problem is this? Is this a problem
for applications built using .NET 1.1 and run with .NET 2.0 ?

What about applications developed using .NET 2.0? Read that they
cannot run on previous runtime versions, so no danger of it using
version 1.0 or 1.1 I suppose. What about .NET 2.0 app on .NET 3.0 or
3.5 runtime? Any compatibility issues there?

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