AbstractThe nauplius is the earliest free-living stage in the development of most crustaceans, except in the majority of the Malacostraca. Several character states of the nauplius larva are used as constitutive for the Crustacea as a whole. The nauplius shows the following structural characters: a median (nauplius) eye, at least three pairs of head appendages (first and second antennae, where the second antenna bears an arthrite; mandibles), a posteriorly directed fold (the labrum) extending over the mouth, and a cephalic (=nauplius) shield. Extant taxa such as the Cephalocarida, Branchiopoda, Ostracoda, Mystacocarida, Copepoda, Cirripedia, Ascothoracida, Rhizocephala, Facetotecta, Euphausiacea, and Penaeidea are known to develop free-living nauplii. Other Crustacea show at least some vestige of an ‘egg-nauplius’ during embryonic development. The diversity of nauplii belonging to major crustacean taxa is briefly described, and a key to these nauplii is provided. The key is also available in digital format, as a JAVA program capable of being modified and expanded as new information arises. The programming structure allows uses in dichotomous or multi-branching formats.