Two new volumes of Methods in Enzymology continue the legacy of this premier serial with quality chapters authored by leaders in the field. Circadian Rhythms and Biological Clocks Part A and Part B is an exceptional resource for anybody interested in the general area of circadian rhythms. As key elements of timekeeping are conserved in organisms across the phylogenetic tree, and our understanding of circadian biology has benefited tremendously from work done in many species, the volume provides a wide range of assays for different biological systems. Protocols are provided to assess clock function, entrainment of the clock to stimuli such as light and food, and output rhythms of behavior and physiology. This volume also delves into the impact of circadian disruption on human health. Contributions are from leaders in the field who have made major discoveries using the methods presented here.