This comprehensive and cogent survey examines the subject of embryonic determination, the set of processes fundamental to animal development by which cells in the early embryo acquire different developmental capabilities. Examining both the classical literature and the newer, molecular findings, the author summarizes the current state of our understanding of determination and poses key questions for the future. He begins with a consideration of how much spatial pattern is already laid down when the egg forms inside the mother, and ends just before the formation of visible organs. Within these limits he also considers evidence obtained by a variety of techniques, both experimental and biochemical, derived from the embryos of a variety of animal groups. This is a suitable text for upper level undergraduates, graduate students and researchers in developmental biology.