In considering the development of computer-assisted instruction (CAI) systems over the past eleven years, one can see a pattern of interaction between advances in computer hardware and software and continuing efforts to solve the basic problems of CAI: problems of achieving a natural-language capability, of keeping the cost low, and of making coding and editing easier. As new equipment and the higher level languages became available, solutions to one or another of these basic problems have become feasible. Now, in the fourth generation of CAI, we have on line, time-sharing capabilities and a powerful and flexible language that embodies both program logic and instructional content. This pattern gives some hint of how things will proceed in the future: through a continuing iteration between technology--in the forms of the mini-computer on the one hand and the extremely large time-share facility on the other--and further efforts to solve the more complicated needs that face us now. (JY)