Testing Concurrent Systems: A Formal Approach. J. Tretmans. In: J.C.M. Baeten, S. Mauw, (eds.), CONCUR'99, pp. 46-65. Lecture Notes in Computer Science 1664, Springer, 1999. This paper discusses the use of formal methods in testing of concurrent systems. It is argued that formal methods and testing can be mutually profitable and useful. A framework for testing based on formal specifications is presented. This framework is elaborated for labelled transition systems, providing formal definitions of conformance, test execution and test derivation. A test derivation algorithm is given and its tool implementation is briefly discussed.