The architecture of an embryo results from complex molecular interactions in time and space. The secrets of these processes are yielding quickly to genetic and cellular dissection in flies, mice and other species, and finding application to human embryology. This volume presents a survey of the induction of axes, control of cell migration, and the development of the nervous system, limbs, wings and other organs, seen from the perspective of 61 investigators. It is completed by a summary that charts the future of this dynamic field.