Abstract. Compositional testing concerns the testing of systems that consist of communicating components which can also be tested in isolation. Examples are component based testing and interoperability testing. We show that, with certain restrictions, the ioco-test theory for conformance testing is suitable for compositional testing, in the sense that the integration of fully conformant components is guaranteed to be correct. As a consequence, there is no need to re-test the integrated system for conformance. This result is also relevant for testing in context, since it implies that every failure of a system embedded in a test context can be reduced to a fault of the system itself.