The genus Nehalennia Selys is revised and redefined. It is demonstrated that Argiallagma Calvert is a synonym of Nehalennia Sel. The Brazilian N. selysi Kirby is redescribed and assigned to minuta Sel., as a subspecies. A lectotype is designated ($: Sao Joao del Key, Minas Gerais, Brazil; Coll. Selys, Inst. R. Sc. Nat. Belg., Brussels). The genus is closely related to other genera with an angulate Irons. It actually includes six species and one subspecies, which are tentatively arranged into a "phylogenetic tree”, based on differences and similarities of morphological characters (larval and adult). The penes and the male anal appendages of all species are figured. A new interpretation of the superior appendages is given and a terminology for both the superiors and the inferiors is proposed. Figures of the female prothorax of all species are provided, as well as drawings of other important structures and colour patterns. For the seven taxa a list of synonyms is given. In the case of N. minuta and pallidula Calv. a more exhaustive bibliography is added. Measures and venational features (males) are presented, as well as a description of the habitat preferences and dates on the distribution.