Giant clams, the largest living bivalves, play important ecological roles in coral reef ecosystems and provide a source of nutrition and income for coastal communities, however, all species are under threat and intervention is required. This chapter reviews the distribution, abundance and conservation status as a contribution to the protection, rebuilding and management of declining populations. It aims to synthesize the taxonomy of giant clams and their global distribution and outlines the information available on their exploitation and the laws that protect them. Giant clams are distributed along shallow coasts and coral reefs from South Africa to the Pitcairn Islands, and from southern Japan to Western Australia. Collectively, giant clams can increase topographic relief of coral reefs, act as reservoirs of zooxanthellae, and potentially counteract eutrophication via water filtering. Giant clams have been utilized by humans for millennia. Giant clams are morphologically derived cardiids which have evolved an obligate symbiotic association with photosynthetic dinoflagellate algae.