.NET

.NET 4.0 will not run .NET 3.x or earlier applications properly

.NET 3.5 will run .NET 3.0 & .NET 2.0 applications, but not .NET 1.1

.NET 3.0 will run .NET 2.0 applications, but not .NET 1.1

.NET 2.0 will not run .NET 1.1

.NET 4.0 installs side-by-side with .NET 3.5/3.0/2.0 and with .NET 1.1

.NET 3.5, 3.0, and 2.0 do not install side-by-side with each other

.NET 3.5/3.0/2.0 installs side-by-side with .NET 1.1