The .NET Framework supports both backward and forward compatibility. In the context of the .NET Framework, backward compatibility means that an application created using an early version of the .NET Framework will run on a later version. Conversely, forward compatibility means that an application created using a later version of the .NET Framework will run on an earlier version.
The .NET Framework provides a high degree of support for backward compatibility. For example, most applications created using version 1.0 will run on version 1.1. The .NET Framework also supports forward compatibility. However, for forward compatibility you might need to modify an application so that the application runs as expected. For both backward and forward compatibility, a change to the .NET Framework that helps improve security, correctness, or functionality might also raise compatibility issues.
For more information about compatibility and for a list of the public API modifications to the class library for version 1.1 that might affect the compatibility of your application, see Compatibility Considerations and Version Changes.