The Windows Template Library (WTL) is an Object-oriented programming Win32 encapsulation C plus plus Library (computer science) by Microsoft. The WTL supports an Application programming interface for use by programmers. It was developed as a light-weight alternative to Microsoft Foundation Classes. WTL extends Microsoft s Active Template Library, another lightweight API for using Component Object Model and for creating ActiveX controls. Though created by Microsoft, it is unsupported.

In an uncharacteristic move by Microsoft—an outspoken critic of open source software—they made the source code of WTL freely available. Releasing it under the open-source Common Public License, Microsoft posted the source on SourceForge, an Internet open-source repository. The SourceForge version is 7.5.

Being an unsupported library, WTL has little formal documentation. However, most of the API is a direct mirror of the standard Win32 calls, so the interface is familiar to most Windows programmers.

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