Unlike most scientific books on circadian rhythms, this book focuses on mechanistic understanding of the clocks themselves and is about the inner machinery of circadian clocks. One emphasis will be on new insights into clock mechanism derived from structures of circadian clock proteins solved by crystallography and NMR. The approach for each chapter will be to introduce the system (e.g., mammalian, insect, bacterial, etc), then describe the relevant clock proteins, their interactions, and mechanisms that control those interactions, using concepts such as allostery, phosphorylation, conformational dynamics, and energetics.