Renewed interest in the developmental basis of organismal complexity, and the emergence of new molecular tools, is improving our ability to study the evolution of metazoan body plans. The most substantial changes in body-plan organization occurred early in metazoan evolution; new model systems for studying basal metazoans are now being developed, and total-genome-sequencing initiatives are underway for at least three of the four most important taxa. The elucidation of how the gene networks that are involved in axial organization, germ-layer formation and cell differentiation are used differently during development is generating a more detailed understanding of the events that have led to the current diversity of multicellular life.