The impetus for writing this book was born out of a realisation that expertise in calorimetric techniques relied for its distribution largely on chance conversations between a limited ‘club’ of practitioners. Many research workers in medicine and other fields of biology have embarked on calorimetry in the mistaken belief that measurement of oxygen consumption and hence energy expenditure is a very straightforward process. It is almost certain that results and conclusions which have been reported from some calorimetric systems are incorrect. There has been no comprehensive text-book, and newcomers to the field have usually acquired knowledge by visiting establishments where calorimeters already exist. Sub-sequently, they have based their own equipment on what they have seen, sometimes unaware that a quite different form of calorimeter might be better suited to their particular needs. Furthermore, practitioners in human calorimetry and farm animal calorimetry seldom meet and have tended to develop their ideas along different lines. Complete systems for calorimetry were, and indeed still are, rarely available commercially and some commercial instruments are of poor accuracy; some indeed are based on incorrect scientific principles. Whether buying a complete system or building one up from components, it is necessary to have a full understanding of the basic principles and of the many sources of error that must be guarded against. We hope that this book will be useful to teachers and students as well as those embarking on calorimetry. The extensive treatment that we have given to the measurement of gas concentrations and flow-rates should also be useful to respiratory physiologists.