Abstract: A system is disclosed for generating test data for testing logic circuits. It has a store storing a respective Logic List for each of the basic logic circuit types in the form of a list of logic states identifying terminals of the logic circuit to which specified data inputs are to be applied and terminals at which specified data outputs are expected.Another store generates connection information relating to a particular logic circuit under test and specifies any external conditions applied to that circuit by means of its terminals which modify its operation. Means are provided which then modify the logic states of the Logic List in dependence on the connection information, so as to produce a final set of test data which can be used immediately, or subsequently, to carry out the tests on the particular logic circuit.