Physiology is the science of life, and sets out to understand how living things work and what makes them distinct from the non-living. It considers how our bodies are supplied with energy, how they maintain their internal parameters, the ways in which we gather and process information, the ways we take action, and the creation of new generations. This Very Short Introduction explores the field of human physiology, considering how the body works, senses, reacts, and defends itself. As Jamie A. Davies shows, human life (and indeed, all life) is sustained by the interplay of a wide variety of physiological mechanisms and principles. He discusses the physiological experiments and research undertaken to understand these processes, and analyzes the ethical issues involved. He also considers the evolution of the scientific field itself, showing how enhanced understandings of physiological knowledge can help inform medical research and care.