The common name blenny is ambiguous at best, as it has been applied to several families of perciform marine fishes all sharing similar morphology and behaviour. There are six families considered "true blennies", all grouped together under the suborder Blennioidei; its members are referred to as blennioids. There are approximately 833 species in 130 genera within the suborder.

Blennioids are generally small fish, with elongate bodies (some almost eel-like), relatively large eyes and mouths. Their dorsal fins are often continuous and long; the pelvic fins typically have a single embedded spine and are short and slender, situated before the pectoral fins. The tail fin is rounded. The blunt heads of blennioids often possess elaborate whisker-like structures called cirri. As generally benthic fish, blennioids spend much of their time on or near the sea floor; many are reclusive and may burrow in sandy substrates or inhabit crevices in reefs, or even empty mollusk shells.

These fish are superficially quite similar to members of the goby and dragonet families, as well as several other unrelated families whose members have occasionally been given the name "blenny".

The six "true blenny" families are as follows:

• Family Blenniidae: Combtooth blennies (including the "sabre_toothed blennies")

• Family Chaenopsidae: Pike-, tube- and flagblennies

• Family Clinidae: Clinids (including the giant kelpfish)

• Family Dactyloscopidae: Sand stargazers

• Family Labrisomidae: Labrisomids

• Family Tripterygiidae: Threefin blennies (or triplefin blennies)