Periwinkle is a common name for any of the edible intertidal snails of the genus Littorina. Periwinkles are represented by 6 species in Canadian coastal waters. Littorina littorea, the common periwinkle, was introduced in 1850 to the East Coast from western Europe where it is a popular food. The northern yellow periwinkle (L. obtusata) is another Atlantic coast species, which is usually found associated with rockweeds. Littorina scutulata, the checkered periwinkle, is a dominant species in British Columbia's coastal intertidal region, where it nestles among barnacles in upper littoral zones. Where its southern range extends into California, L. scutulata is in competition with the eroded periwinkle, L. planaxis.

Species of Littorina usually occupy the upper reaches of the rocky intertidal zones where they graze on encrusted algal material by means of a radula, a ribbonlike organ covered by many hundreds of minute teeth. This feeding device emerges from a snoutlike head which supports a pair of tentacles each bearing a single eye. The littorines are a relatively advanced group of prosobranch gastropod molluscs with separate sexes and internal fertilization. Some species release pelagic egg cases from which larvae hatch; others brood their eggs, giving birth to larvae or little snails.