The purpose of an instructional system is to bring about specific changes in the capabilities of individuals; that is, their potential performance of certain intellectual or motor activities. Achievement and attitudes of learners entering into instruction constitute the input of the system, while their change capabilities are the output. Because there are different ways to ask people to do a specific thing, designers may use different instructional activities for the same system function. Instructional theory describes the reality of instructional systems and theory must describe reality in such a way that hypotheses can be inferred and tested. The three corporate sources for knowledge about instruction are practice, empirical research, and deductive theory. It is questionable, with all the variables and different methods available, that there will ever be one integrated theory of instructional design developed, based solely on knowledge of human learning and functioning as a useful tool for designers of all types of instruction, or a yardstick for criticizing instructions developed by others. (CB)