Since the discovery of circadian rhythms in cyanobacteria in the late 1980s, the field has exploded with new information. The cyanobacterial model system for studying circadian rhythms, Synechococcus elongatus, has allowed a detailed genetic dissection of the bacterial clock due to the methods currently available in molecular, structural, and evolutionary biology. This book addresses multiple aspects of bacterial circadian programs: the history and background of the cyanobacteria and circadian rhythms in microorganisms, the molecular basis, structure, and evolution of the circadian clock, entrainment of the oscillator with the environment and the control of downstream processes by the clock, the demonstration of adaptive significance and the prokaryotic clock remarkable stability, and mathematical and synthetic oscillator models for clock function. Experts in the field provide a timely and comprehensive review and a stepping-stone for future work on this amazing group of microorganisms and timing.