The common name mussel is used for members of several different families of clams (bivalve molluscs) from both saltwater and freshwater habitats. "Mussel" is a loose and inaccurate term, but it has historically been applied to those families of clams where the shell is longer than it is wide, being wedge-shaped or assymetrical-looking, and where the external color of the shell is dark blue or brown, as opposed to the more globular lighter-colored families of bivalves.

Marine mussel species live in intertidal and subtidal areas along coastlines worldwide. Freshwater mussel species inhabit lakes, ponds, rivers, creeks, canals, and similar habitats.

Freshwater mussels (several allied families, the largest being the Unionidae) and saltwater mussels (family Mytilidae) are not closely related at all. They are taxonomically grouped in different subclasses, despite some superficial similarities in appearance. For example, the freshwater Zebra mussels and their relatives in the family Dreissenidae somewhat ressemble marine mussels such as Mytilus species, and they can live attached to rocks and other hard surfaces in a manner similar to marine mussels, but they are classified with the Heterodonta, the taxonomic group which includes most of the bivalves that are commmonly referred to as "clams".