Abstract: A compatibility service monitors programs to detect compatibility issues. The compatibility service provides users, such as developers, with notifications of compatibility issues a program may experience when the program executes on a particular device and/or uses a particular operating system. In some configurations, the compatibility service detects compatibility issues by performing tests on programs using a testing service. The compatibility service may also receive notifications of incompatibilities from different sources. For example, the compatibility service may receive notifications of incompatibilities from other users (e.g., developers), web sites, and the like. The compatibility service may proactively notify developers of compatibility issues for their programs. For example, one developer may request to be notified of all detected compatibility issues, whereas another developer might request to be notified of compatibility issues that are above some specified severity level of incompatibility.