Comparative developmental physiology is a growing discipline examining a diversity of organisms as they transform from single cells to mature, reproductive individuals. This collection of original, innovative essays emerged from a Roundtable on Comparative Developmental Physiology held in Glen Rose, Texas in the summer of 2002. This meeting brought together investigators studying the physiology of developing animals in an effort to identify the field's potential contributions to biology. The participants honed in on common emerging themes and future goals, which are reflected in the chapters within. The nascent community of comparative developmental physiologists was challenged to amplify the power of data collection and tool development by focusing on a few select model organisms, while still employing the power of the broader, more traditional comparative approach. Evolution has provided comparative developmental physiologists with remarkable biological diversity, which they have used to investigate a broad range of questions critical for understanding how life works. This goes beyond the basic nuts and bolts of cellular mechanisms to the functional whole, from the mechanistic level to behavior within and between organisms. The union of developmental biology with the breadth of comparative physiology holds much promise for a deeper understanding of evolutionary processes.