Four of the major animal systems studied for the mechanisms of their early embryonic development are treated in this volume. The articles address the specific questions studied in the various systems, discuss the fundamental questions raised by the particular organism and explain the techniques used to find answers to these questions. Questions of patternformation, early organogenesis and the genetics of the early development arecovered as well as the question of parental imprinting phenomena in mammals which are important for the early differentiation. The development of the mouse, Drosophila, Caenorhabditis and the zebrafish is emphasized by leading experts of their fields, and current problems in each system are exposed. For the zebrafish the advantages of this new system for developmental biology studies are summarized and discussed in their values, while in the other system the emphasis is laid on one of the actual field of research.