Concepts and technologies are described herein for cross-platform application programming interfaces for applications. In accordance with the concepts and technologies disclosed herein, the cross-platform APIs support interoperability and extensibility with various application programs using a single language and/or set of APIs. Extensions authored for a particular application program may be used with other application programs that expose the same or a similar set of APIs, even if other object models or APIs associated with the application programs differ from one another. Thus, the APIs support cross-platform, cross-device, and cross-version compatibility of extensions.