Do you need both .net 3.5 and 4.0 installed or just .net 4?

Question

to run both 3.5 and 4.0 web apps?

Answer 1

To add one more point...

All Frameworks prior to 4.0 [4.0 CLR] (yes, even 3.5.1) are based on the 2.0 CLR. Microsoft has made every attempt to make 4.5 [4.5 CLR] a "seamless" in place upgrade (or service [if not feature] pack).

Like another astute commented stated above, if you have dependencies for 3.5.1 that can't be transitioned, choices are a bit more limited. Does anyone know if the Entity and LINQ stacks have been fully transitioned from the 3.5.1 Framework? I'm assuming that it has, but not sure.

Another note... Simply installing 4.5 on some servers... with MOSS2K7, SP2010 services may be fun... ;-o