An elegant analysis of how animals work and function. Professor Schmidt-Nielsen's incisive account gives a clear understanding of comparative physiology in relation to body size, form and function, energy supply, and environment. The author is concerned with principles. For example, he explains how difficult it may be to lose heat and water from the respiratory tract. This leads to a consideration of the mechanism of panting as a means of heat loss. The author describes the centuries-old problem of how birds breathe, which now has been solved in his laboratory. He then discusses energy expenditure for swimming, running, and flying, and the effects of activity on heat balance. The ability of mammals to maintain different parts of the body at different temperatures is explained on the basis of counter-current heat exchange; a related mechanism permits the fast-swimming tuna to enjoy some of the advantages of being warm-blooded. The problems raised by being small in size, or large, are considered in detail. It is shown that many physiological variables can be placed on a scale which permits the derivation of non-dimensional numbers to describe the interrelations between different parameters. This interesting and stimulating account was written primarily for students, but since it brings together and synthesizes much new and up-to-date information it will interest all biologists and physiologists.