Onchidorids possess a broad mantle that completely covers the entire body, is oval in shape and embedded with spicules. They generally have a flattened appearance.
The exception is the Diaphorodoris genus which instead has a much reduced mantle by comparison and therefore the tail of the body protrudes significantly. The mantle in most is thick and covered with tubercles or long papillae.
The large lamellate rhinophores are able to contract somewhat into shallow pockets. The gills are simple, pinnate and form a circle around the anus on the dorsal midline posteriorly and are only able to partially retract. Diaphorodoris possesses a common gill pocket.
Encrusting bryozoans are the food of nearly all onchidorids. Most are temperate in distribution there being only a few tropical species reported.
The lifecycle is generally considered to be annual.