If the biosphere really is a single coherent system, then it must have something like a physiology. It must have systems and processes that perform living functions. In this text, the author describes the environment that enables the biosphere to exist, various ways of looking at its anatomy and physiology, the major biogeographical regions such as rainforests, deserts, and tundra, the major substances the biosphere is made of, and the chemical cycles that keep it in balance. He then looks at the question of whether there are any long-term trends in the Earth's evolution, and examines the role of humanity in Gaia's past and future. This book describes, for scientists, students, and lay readers alike, the theory's basis in science.