A basic tenet of present day biophysics is that flows in biological systems are causally related to forces. A large and growing fraction of membrane biophysics is devoted to an exploration of the quantitative relationship between forces and flows in order to understand both the nature of biological membranes and the processes that take place on and in these membranes. This is why the discussion of the nature of diffusion is so important in any formal development of membrane bio­ physics. This was equally true twenty years ago when tracers were just beginning to be used for the measurement of membrane processes. We turned naturally to the great treatises on the physics of diffusion and the flow of heat where, to be sure, we could dig out the information that was needed. It was a great joy then to come across this masterful and schol­ arly discussion on diffusion written for biologists of a physical tum of mind by MERKEL JACOBS. Here were to be found not only the equations that were basic to our knowledge, but also a careful, accurate and logical explanation, both of the physical principles and the mathematical steps. It soon became apparent that we could not keep that one volume of Ergebnisse der Biologie on indefinite loan from the library, and we then found, by good fortune, a remaindered copy of this particular issue. It has become a well-thumbed and treasured possession of the laboratory.