This paper presents a testing theory that is parameterised with assumptions about the way implementations communicate with their environment. In this way some existing testing theories, such as refusal testing for labelled transition systems and (repetitive) quiescence testing for I/O automata, can be uni ed in a single framework. Starting point is the theory of refusal testing. We apply this theory to classes of implementations which communicate with their environment via clearly distinguishable input and output actions. These classes are induced by making assumptions about the geographical distribution of the points of control and observation (PCO's) and about the way input actions of implementations are enabled. For speci c instances of these classes our theory collapses with some well-known ones. For all these classes a single test generation algorithm is presented that is able to derive sound and complete test suites from a speci cation.