The superclass Hydrozoa of the phylum Cnidaria comprises 3,702 species currently regarded as valid, ascribed to three heterogeneous classes. The 134 species of the class Automedusa have simple life cycles with medusae and no polyp stage, and are divided into three subclasses: Actinulidae, Narcomedusae, Trachymedusae. The 3,567 species of the class Hydroidomedusa have complex life cycles with polyps producing medusae through a medusary nodule, and are divided into five subclasses: Anthomedusae, Laingiomedusae, Leptomedusae, Limnomedusae, Siphonophorae. The single species of the class Polypodiozoa has a complex life cycle, involving endocellular parasitism. Diagnoses and keys are presented for all supraspecific taxa, and lists of included species are given for each genus. The diagnostic characters and character states in the superclass are described. A glossary summarizes definitions of all technical terms referred to in the superclass. References to all original species descriptions are given. Zhangiella nom. nov. pro Platystoma Zhang, 1982, non Meigen, 1803 Pseudosolanderiidae Bouillon & Gravier-Bonnet, fam. nov.