From the results of studies conducted from the 1960s, three patterns of ontogeny of postembryonic osmoregulation in crustaceans may be distinguished: (1) osmoregulation varies little with developmental stage and the adults are often weak regulators or osmoconformers;(2) the adult type of osmoregulation is established in the first postembryonic stage; (3) metamorphosis marks the appearance of the adult type of osmoregulation. The occurrence of osmoregulation is based on efficient ionic regulation (mainly of Na+ and CI−) and increased levels of Na+-K+ATPase activity. Osmoregulatory epithelia may appear at different times of development and at different locations of the branchial chamber. These events are inter-related, and are correlated with changes in salinity tolerance. In species the eggs of which are exposed to the external medium, either embryos are osmotically protected by the egg envelopes, or the egg membranes and/or the embryos acquire the ability to osmoregulate. When embryos develop in incubating pouches enclosed in the female's body, where they are osmotically protected, they may also become progressively able to osmoregulate. Throughout the development of crustaceans, osmoregulation appears as a key adaptative factor to one or several habitats.