The aim of this book is to shed light on one of the most fundamental processes of life in the various lineages of animals: respiration. It provides a certain background on the physiological side of respiration, but it clearly focuses on the morphological aspects. In general, the intention of this book is to illustrate the impressive diversity of respiratory faculties (form–function complexes) rather than serving as an encyclopaedic handbook. It takes the reader on a journey through the entire realm of animals and discusses the structures involved in gas exchange, how they work, and most importantly, how all of this may be connected on an evolutionary scale. Due to the common problem, namely oxygen uptake and carbon dioxide release, and the limited number of solutions, basically surface area, barrier thickness, and physical exchange model of the respiratory organ, it is not surprising that one finds a huge number of convergences. These include, for instance, the repeated origin of tubular tracheae among several lineages of arthropods, similar lung structures in snails and amphibians, and counter-current exchange gills in bivalves and fish. However, there are certain phylogenetic constraints evident and the respiratory faculty appears as a yet to be adequately exploited source of information for systematic considerations. The ultimate goal of this book is to stimulate further research in respiratory biology, because a huge number of questions remain to be tackled on all levels, ranging from molecular through functional to especially the evolutionary aspects.