A new voice in the nature-nurture debate can be heard at the interface between evolution and development. Phenotypic integration-or, how large numbers of characteristics are related to make up the whole organism, and how these relationships evolve and change their function-is a major growth area in research, attracting the attention of evolutionary biologists, developmental biologists, and geneticists, as well as, more broadly, ecologists, physiologists, and paleontologists. This edited collection presents much of the best and most recent work the topic.