Book excerpt: Mollusca is one of the largest phyla in the animal kingdom, over 50,000 species having been described, thus comprising more than a half of the marine species. Molluscs are scientifically and economically important invertebrates, representing 8% of all captured marine species. One of the six molluscan classes is Bivalvia, which is composed of both marine and freshwater animals enclosed in shells with two valves. Among these, the groups of greatest economic importance are oysters, scallops, mussels and clams. This book deals with clams in the broad sense of the term, since there are several bivalve families falling into the "clam" category. The selection of the families is primarily based on their respective economic importance, in terms of fisheries and aquaculture production. These selected families are Veneridae (venus shells), Mactridae (surf clams), Donacidae (wedge shells), Myidae (softshell clams), Pharidae and Solenidae (razor clams). It must be remarked that, in some chapters or sections within them, species belonging to other families of clams are cited.