The study of the normal function of the animal and human organisms and of the diseases which disturb that normal func­ tion is largely the study of control mechanisms. These control mechanisms are essential for the survival of an organism in a more or less hostile environment. In many ways they clearly resemble the control mechanisms devised by electronic engin­ eers for running machinery of all kinds and there are many remarkable parallels between biology and engineering. However, it should not be forgotten that the biological systems were on the scene first and that the engineering is a parallel and independent development. It is therefore perhaps a pity that in recent years the study of biological control systems has tended to be dominated by mathematicians and engineers who have moved from these more precise disciplines into biology. As a consequence of this dominance, one often gets the impression that the principles of biological control can be understood only after one has undergone a rather high-powered course in elec­ tronic control theory. It often seems to be assumed that it is electronics which must do all the teaching while biology and medicine must do all the learning. In fact I suspect that biolo­ gical control mechanisms are considerably more sophisticated than anything yet available in the world of the physical sciences and that in the long run biology will teach more to control engineers than vice versa.