A membrane clock model involving ions and ion transport is compatible with the limit cycle concept, biochemical studies, and the idea of phase shifting by ion gating. It provides an explanation for temperature compensation, and, by assuming that oscillating ion concentrations regulate the many different biochemical and physiological rhythms, it explains how diverse systems can be controlled by a single oscillatory mechanism. How ions control ion transport and which membranes are involved in the clock are two questions we have begun to explore.