Interest in comparative acid-base physiology has considerably grown during last decades even in the absence of major technical or conceptual advances. This is firstly because it has become clear that the extracellular acid-base state reflects the performance of many exchange functions at the organism level: respiration and ventilation of the gas exchange surfaces, metab­ olism, iono-and osmoregulation. Such functions are much in­ fluenced by ambient conditions, and the measurement of acid­ base parameters thus provides useful information about the organism's responses to environmental challenges. Secondly, many processes at the molecular level are now known to be pH­ sensitive, and acid-base regulation thus appears to be a major requirement for the functional integrity of cells and organisms. How extracellular acid-base balance can be maintained in a wide variety of animals living in different conditions is the sub­ ject of this book. The approach is comparative and environ­ mental throughout. All body fluids share similar buffer proper­ ties, and common physicochemical principles apply to any acid­ base system. However, in accord with differing designs and con­ straints along animal evolution, varying effector organs and mechanisms are at work to maintain an appropriate acid-base state in the organism. Particular emphasis is placed on the fun­ damental differences between water and air breathers and on the acid-base and respiratory problems arising at the transition from an aquatic to a terrestrial life. Also the complex array of factors influencing the acid-base state in water-dwelling animals is thoroughly discussed.