.NET apps will not run against older versions of the Framework but they will run against newer versions, so your version 1.1 app will run if only version 2.0 is present. That said, there is no guarantee that it will run without error. Unless you have fully tested your app against version 2.0 then it is foolhardy to assume that it will work. Note that there is no reason not to install version 1.1 even if version 2.0 is required for other applications because the .NET FRamework has been specifically designed with side-by-side installation in mind. You can have as many versions of the Framework installed as you like and they will not affect or interfere with each other.