The Windows API is the name given by Microsoft to the core set of application programming interfaces available in the Microsoft Windows operating systems. It is designed for usage by C/C++ programs and is the most direct way to interact with a Windows system for software applications. Lower level access to a Windows system, mostly required for device drivers, is provided by the Windows Driver Model in current versions of Windows.

A software development kit (SDK) is available for Windows, which provides documentation and tools to enable developers to create software using the Windows API and associated Windows technologies.


The Windows API has always exposed a large part of the underlying structure of the various Windows systems for which it has been built to the programmer. This has had the advantage of giving Windows programmers a great deal of flexibility and power over their applications. However, it also has given Windows applications a great deal of responsibility in handling various low-level, sometimes tedious, operations that are associated with a Graphical user interface.

Charles Petzold, writer of various well read Windows API books, has said: "The original hello-world program in the Windows 1.0 SDK was a bit of a scandal. HELLO.C was about 150 lines long, and the HELLO.RC resource script had another 20 or so more lines. (...) Veteran C programmers often curled up in horror or laughter when encountering the Windows hello-world program."[9]. A hello world program is a frequently used programming example, usually designed to show the easiest possible application on a system that can actually do something (i.e. print a line that says "Hello World").

Over the years, various changes and additions were made to the Windows Operating System, and the Windows API changed and grew to reflect this. The Windows API for Windows 1.0 supported fewer then 450 function calls, where in modern versions of the Windows API there are thousands. However, in general, the interface remained fairly consistent, and an old Windows 1.0 application will still look familiar to a programmer who is used to the modern Windows API.[10]

A large emphasis has been put by Microsoft on maintaining software backwards compatibility. To achieve this, Microsoft sometime even went as far as supporting software that was using the API in a undocumented or even (programmatically) illegal way. Raymond Chen, a Microsoft developer who works on the Windows API, has said that he "could probably write for months solely about bad things apps do and what we had to do to get them to work again (often in spite of themselves). Which is why I get particularly furious when people accuse Microsoft of maliciously breaking applications during OS upgrades. If any application failed to run on Windows 95, I took it as a personal failure