Goal-seeking behaviour, once a great mystery, is now beginning to be understood. In its simplest forms it is, in fact, understood to-day almost completely. Thus the theory of the simple regulator, such as the thermostat, not only includes an extensive repertoire of techniques, but the elementary principles, of the necessity for negative feedback for instance, are becoming scientific commonplaces. In getting to know, however, about these simple systems and their principles, we should not make the mistake of thinking that there is nothing more to be learned. On the contrary, in real life many an important goal is to be achieved only through some quite complex pattern of behaviour, a pattern for which the simple concept of “negative feedback” is quite inadequate. It is of these more complex patterns that I wish to speak to-day.