The fundamental function of buildings today is to provide safe and healthy shelter. For the fortunate they also provide comfort and delight. In the 20th Century comfort became a 'product' produced by machines, run on cheap energy. In a world where fossil fuels are becoming scarcer and more expensive, and the climate more extreme, the challenge of designing comfortable buildings requires a new approach. This timely book provides just that. It explains, in a clear and comprehensible manner, how we stay comfortable by using our bodies, our minds and our buildings, and their systems, to adapt to indoor and outdoor conditions which change with the weather and the climate. The book is in two sections. The first introduces the Principles on which the theory of adaptive thermal comfort is based. The second explains how to use field studies to measure thermal comfort in Practice. Architects have gradually passed responsibility for building performance to service engineers who are largely trained to see comfort as that very 'product', using machines and outdated comfort models which produce more energy and provide less comfort. A growing international consensus now calls for low energy buildings. This means designers must first produce robust, passive structures that provide their occupants with many opportunities to change their environments to suit their needs. Ventilation using free, natural energy should be preferred and mechanical conditioning only used when the climate demands it. This book outlines the theory of adaptive thermal comfort that can inform and guide such building designs. It should be required reading for all students, teachers and practitioners of architecture, building engineering and management -- for all who have a role in producing 21st Century adaptive, low carbon, comfortable buildings.