This book is an introduction to a basic property of life, one mostly unknown to science and the public until the latter half of the last century: Humans, plants, and animals have within their bodies a kind of clock that synchronizes much of what they do throughout their lives to the time of day and the seasons, and in the case of the sea-dwelling organisms, the tides. This timepiece performs its service autonomously - it rules silently within us without us giving a thought to it. Three chapters are devoted to the human clock: its disruptive action in transmeridional travel and shift work, its oversight in most every aspect of our physiology, and how doctors being aware of its action can save lives. Other major subjects describe the role in piloting birds in homing and migration, guiding the seasonal reproduction of plants and animals, and its influence on shore dwellers. The book closes with a description of the clockworks' escapement.