Composition system thread techniques are described. In one or more implementations, a composition system may be configured to compose visual elements received from applications on a thread that is executed separately than a user interface thread of the applications. As such, the composition system may execute asynchronously from a user interface thread of the application. Additionally, the composition system may be configured to expose one or more application programming interfaces (APIs) that are accessible to the applications. The APIs may be used for constructing a tree of objects representing the operations that are to be performed to compose one or more bitmaps. Further, these operations may be controlled by several API visual properties to allow applications to animate content within their windows and use disparate technologies to rasterize such content.