The use of indirect calorimetry to measure the heat production of men and animals has increased rapidly since the pioneering work of Lavoisier. Measurement of the consumption of oxygen and production of carbon dioxide are the basis for the measurement of heat production. Today, applications of indirect calorimetry are available in many species. Combining these measurements with accurate climate control, recording of physical activity and feed intake, use of stable isotopes and sophisticated modelling techniques allow scientists to make progress in various research areas.This book provides a scientific basis for indirect calorimetry, dealing with smart ways to design calorimeters, gas measurements and computational techniques to deal with complex data. Novel techniques allow the connection between short term changes in energy expenditure, protein turnover and substrate oxidation, e.g. using stable isotopes. Various applications of indirect calorimetry are addressed, including heat production measurements in growing animals, hatching eggs, companion animals and in animals housed under heat stress conditions. In addition, various ways of measuring methane emissions are discussed.This book is intended for scientists working or interested in calorimetry or metabolism research, or people designing calorimetry systems, opening their eyes for applications they did not yet think of.