Four of the major animal systems studied for the mechanismsof their early embryonic development are treated in thisvolume. The articles address the specific questions studiedin the various systems, discuss the fundamental questionsraised by the particular organism and explain the techniquesused to find answers to these questions. Questions ofpatternformation, early organogenesis and the genetics ofthe early development arecovered as well as the question ofparental imprinting phenomena in mammals which are importantfor the early differentiation. The development of the mouse,Drosophila, Caenorhabditis and the zebrafish is emphasizedby leading experts of their fields, and current problems ineach system are exposed. For the zebrafish the advantages ofthis new system for developmental biology studies aresummarized and discussed in their values, while in the othersystem the emphasis is laid on one of the actual field ofresearch.