Cellular and molecular mechanisms involved in the establishment of the zebrafish embryonic body plan appear strikingly similar to those operating in amphibians or other vertebrates. The specification of embryonic polarity is initiated at early stages of zebrafish development, leading to the establishment of a dorsal blastula signaling center, an equivalent of the Nieuwkoop center, by the midblastula stage. In contrast to Xenopus, fertilization of the zebrafish egg is restricted spatially and occurs at a single sperm entry point located at the future animal pole of the embryo. The initial characterization of a collection of zebrafish mutations affecting early pattern formation provided key genetic evidence for the model of dorsoventral patterning of the gastrula via the antagonistic interactions between the ventral morphogen, BMP-2/4, and its dorsal inhibitors. It is likely that zebrafish mutants also will identify novel components of the early patterning system. It is likely that zebrafish mutants also will identify novel components of the early patterning system. Furthermore, they will be an important tool in the elucidation of the genetic hierarchies regulating this process as well as in the functional analysis of the genes involved.