Microsoft .NET Framework 3.5 SP1 is the runtime that older Windows programs built for .NET Framework 2.0, 3.0 and 3.5 need in order to run.
The Microsoft .NET Framework 3 is the managed-code programming model for Windows. It combines the power of the .NET Framework version 2.0 with newer technologies for building applications with visually compelling user experiences, seamless communication across technology boundaries and support for a wide range of business processes.
What version 3.5 includes. Version 3.5 contains many new features building incrementally on .NET Framework 2.0 and 3.0, and it includes .NET Framework 2.0 Service Pack 1 and .NET Framework 3.0 Service Pack 1. Service Pack 1 (SP1) of version 3.5 is the final, fully updated release of this branch. Installing 3.5 SP1 therefore gives you the 2.0, 3.0 and 3.5 runtimes in one package.
How to install it today. The .NET Framework 3 was included in Windows Vista, where it could be installed or removed through the Windows Features control panel, and the redistributable package was made for Windows XP and Windows Server 2003. On Windows 10 and Windows 11, .NET Framework 3.5 is an optional Windows feature: open "Turn Windows features on or off" and tick ".NET Framework 3.5 (includes .NET 2.0 and 3.0)", or let Windows install it automatically when a program asks for it.
Support. Released on 18 November 2008, .NET Framework 3.5 SP1 is still supported by Microsoft until 9 January 2029. It works side by side with .NET Framework 4.8.1, which is needed for newer programs.