There are many excellent reasons for a chemist to be interested in membrane separations, particularly if he is interested in the biochemistry of larger molecules. In living tissues, with their amazing ability to synthesize delicate, large polymers with unique structures, perform equally amazing separations of closely related solutes of complicated nature, maintain concentration gradients, and otherwise control a local environment, there seems always to be an intricate system of membranes. This has been realized for a long time and the literature dealing with biological membranes has become so voluminous as to be difficult to master. Nonetheless, it seems to fall far short of providing adequate explanations for all the phenomena observed.