If I have .Net 4 installed do I need the previous versions? [Yes]

Aaron Stebner's WebLog - Mailbag: Do I need still need older versions of the .NET Framework on my system after installing the .NET Framework 4?

"Question:

I recently installed the .NET Framework 4 on my system. Afterwards, I looked in Add/Remove Programs, and it shows that I have all of the following versions of the .NET Framework installed on my system:

• Microsoft .NET Framework 1.1
• Microsoft .NET Framework 2.0 SP2
• Microsoft .NET Framework 3.0 SP2
• Microsoft .NET Framework 3.5 SP1
• Microsoft .NET Framework 4 Client Profile
• Microsoft .NET Framework 4 Extended

Do I need any of these older versions of the .NET Framework now that I’ve installed the .NET Framework 4, or can I safely uninstall them?

Answer:

In general, my recommendation is to leave the .NET Framework 2.0 SP2, 3.0 SP2, 3.5 SP1 and 4 installed on your computer.

Unlike previous versions of the .NET Framework, the .NET Framework 4 does not allow an application that was built with previous versions of the .NET Framework to migrate forward and run on it if the previous version is not installed. If you are using any applications that were built with any version of the .NET Framework before version 4, then I recommend leaving both the .NET Framework 3.5 SP1 and the .NET Framework 4 installed.

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I've seen this asked a few times and so wanted to capture and highlight it.