Keys are presented for the identification of marine, benthic, crustacean nauplii belonging to the Ostracoda, Cephalocarida, Mystacocarida, and 15 families of harpacticoid copepods. Characters taken from the biology or habitat of taxa are considered. Key diagnostic morphological characters are illustrated. A key to stages allows the differentiation among the six naupliar stages of the Harpacticoida