This book covers the main aspects of biological rhythms. It focuses on the evolution and basic features of the biorhythms in organisms, deals with the circadian system at the genetic, molecular and cellular levels, and describes the mechanisms involved in the perception and light entrainment of the biological clock in vertebrates and invertebrates. The most important features of the biological clock are summarized on the level of whole organisms, from fish to mammals, and long-term (seasonal) rhythms in plants and higher vertebrates are discussed. Finally, the book concentrates on short-term rhythms, the significance of having a biological clock system in animals living in extreme (Arctic) environments, and on the diversity of circadian responses to melatonin, one of the key endocrine elements involved in the regulation of biological rhythms.