Following a brief overview of the patterns of ontogeny of osmoregulation in postembryonic stages, this review concentrates on the ontogeny of osmoregulation during the embryonic development of crustaceans, particularly in those species living under variable or extreme salinity conditions and whose hatchlings osmoregulate at hatch. Two situations are considered, internal development of the embryos in closed incubating, brood or marsupial pouches, and external development in eggs exposed to the external medium. In both cases, embryos are osmoprotected from the external salinity level and variation, either by the female pouches or by the egg envelopes. The mechanisms of osmoprotection are discussed. During embryonic life, temporary or definitive osmoregulatory organs develop, with ion transporting cells and enzymes such as Na -K ATPase, permitting the embryos and then the hatchlings to osmoregulate and tolerate the external salinity.