This book is about theories and applications of thermosyphons and heat pipes. It discusses the physical phenomena that drive the working principles of thermosyphons, heat pipes and related technologies. Many applications are discussed in this book, including: rationalizing energy use in industry, solar heating of houses, decrease of water consumption in cooling towers, improvement of the thermal performance of industrial and domestic ovens and driers and new devices for heating stored oil and gas in petrochemical plants. Besides, the book also presents heat pipe and thermosyphon technologies for the thermal management of electronic devices, from portable equipment to airplanes and satellites. The first part of the book explores the physical working principles of thermosyphons and heat pipes, by explaining current heat transfer and thermal resistance models. The author discusses the new heat pipe and thermosyphon technologies that have been developed in the last decade for solving a myriad of electronic, environment and industrial heat and thermal problems. The focus then shifts to the thermosyphon technology applications, and the models and simulations necessary for each application including vehicles, domestic appliances, water conservation technologies and the thermal control of houses and other structures. Finally, the book looks at the new technologies for heat pipes (mini/micro) and similar devices (loop heat pipes), including new models for prediction of the thermal performance of porous media. This book inspires engineers to adopt innovative approaches to heat transfer problems in equipment and components by applying thermosyphon and heat pipe technologies. It is also of interest to researchers and academics working in the heat transfer field, and to students who wish to learn more about heat transfer devices.