Isopods of the family Gnathiidae have free-living adults and parasitic juveniles feeding on the blood and tissue fluids of teleost and elasmobranch fishes. When not feeding on fishes, gnathiids are cryptic and widely distributed, especially among marine habitats. Ten genera are recognized: Bathygnathia, Bythognathia, Caecognathia, Elaphognathia, Euneognathia, Gibbagnathia, Gnathia, Monodgnathia, Paragnathia and Thaumastognathia. Among these are 172 known species, the majority in the genus Gnathia. Species descriptions rely on the morphology of adult male gnathiids. When juveniles or females are found, their identification can be difficult, a problem discussed in this review. Several gnathiids adapt well to laboratory culture and life cycles are generally similar, although variations in moulting behaviour, length of cycle and harem formation are observed. Praniza larvae are the feeding stages, and their mouthparts and digestion processes are examined. The effects of feeding on fishes in aquaria, in fisheries and in the wild are reported, and the role of gnathiids as vectors is assessed. Ecological interactions between gnathiid larvae, client and cleaner fishes are summarized. Identification of juveniles, host-finding behaviour, feeding and the digestion processes in larvae, feeding cycles among gnathiids of elasmobranchs, and the role of gnathiids as vectors, are among areas highlighted for further study.