The Microsoft Windows SDK (Software Development Kit) is a software development kit from Microsoft that contains header files, libraries, samples, documentation and tools utilizing the APIs required to successfully develop applications for Microsoft Windows and the .NET Framework 3.0/.NET Framework 3.5. The Windows SDK can be used to write applications that target Windows XP and Windows Server 2003 as well as Windows Vista and Windows Server 2008 while the older Platform SDK can be used to develop applications for Windows 2000, Windows XP and Windows Server 2003. The Windows SDK replaces the Platform SDK, the WinFX SDK and the .NET Framework 2.0 SDK. The package contains over 100 million words of documentation and nearly 1000 samples. Although the SDK itself is available free of cost, obtaining the SDK requires that the copy of Windows be validated.