Freshwater "mussels," "clams," "naiads" or "unios" are members of the Unionoida, an order of bivalve mollusks. From three to seven families are extant, depending on which classification you use. Unrelated to true clams or mussels, they appear to be derived from the marine trigonioideans (see below), a once diverse fossil group represented today by a handful of species. Perhaps 300 Recent species of unionoids occurred in North America. These belong to two families. The Margaritiferidae are few in number and limited in distribution, with only five species in North America. The remaining species are members of the Unionidae. Most of Europe and Asia have species of Unionidae, but North America has more species of unionoids than any other continent. Other families occur on other continents. The Hyriidae, closely related to the Unionidae, are found in South America, New Zealand, and Australia. The Mycetopodidae occur mainly in South America. The Mutelidae are African. These two families are closely related to each other, and may represent an independent invasion of freshwater apart from that of the Unionidae-Hyriidae-Margaritiferidae.

Marine bivalves have independently invaded freshwater many times. Today we have such unrelated freshwater forms as the Sphaeriidae (fingernail clams), Corbiculidae (Asian clam), Dreissenidae (zebra mussels), and others, all occurring together with unionoids. Several fossil groups resembling unionoids arose in the Carboniferous Period, but they do not appear to be ancestral to our Recent species. Unionoids date from at least the Triassic, and had reached a great diversity by the end of the Cretaceous, when most species became extinct. We are now experiencing another explosion of diversity.