Rapidly accumulating evidence suggests that periodic behaviour is not confined to a limited number of cell types but is a common property of most biological systems. The argument for this proposition is presented by systematically cataloguing an atlas of biological and biochemical oscillators with periods of an hour or less. The listing consists of eight principal sections .and includes oscillations in secretory cells, neural oscillators, oscillations in muscle cells and rhythmic behaviour in growth and development. Each entry states the experimental preparation, the periodic event (the observed oscillatory variable), the period and lists references to the experimental literature. Four hundred and fifty experimental papers are cited.