An intriguing class of biological periodicity consists of rhythms with about 24-hour periods occurring at every level of eukaryotic organization. Progress is being made in understanding these rhythms. The six chapters of this work include a brief introduction to circadian (24-hour) rhythms, a survey of circadian organization at the cellular level, and a description of the important microorganisms that have served as experimental models for biochemical analysis. Also considered are relations between cell division cycles and circadian oscillators, as well as some general and theoretical aspects. Where appropriate, parallels are drawn to neuronal oscillators. This volume will introduce and critically appraise modern chronobiology; its extensive illustrations and comprehensive up-to-date bibliography will make it an authoritative reference.