1. A system for generating test data for testing any one of a plurality of logic circuits, comprising a store storing for each of the basic logic circuit types data in the form of a respective list of logic states each logic state identifying one or more particular terminals of the logic circuit to which specified data inputs are to be applied and one or more particular terminals at which specified data outputs are to be produced if the logic circuit is operating correctly, means generating connection information relating to a particular logic circuit (of one of the basic types) to be tested and specifying any external conditions applied to that circuit by means of its terminals, and modifying means operative to modify the logic states in the stored list for that logic circuit in dependence on the connection information whereby to produce the said test data.