Methods, systems, and apparatus, including medium-encoded computer program products, for managing an application execution environment on a computer. One or more aspects of the subject matter described in this specification can be embodied in one or more methods of managing an application execution environment, the one or more methods including: identifying which of multiple different versions of the application execution environment are installed on a computer; and managing the different versions of the application execution environment on the computer; wherein the identifying and the managing are performed using one or more of the different versions of the application execution environment.