Giant clams (Hippopus and Tridacna species) are thought to play various ecological roles in coral reef ecosystems, but most of these have not previously been quantified. Using data from the literature and our own studies we elucidate the ecological functions of giant clams. We show how their tissues are food for a wide array of predators and scavengers, while their discharges of live zooxanthellae, faeces, and gametes are eaten by opportunistic feeders. The shells of giant clams provide substrate for colonization by epibionts, while commensal and ectoparasitic organisms live within their mantle cavities. Giant clams increase the topographic heterogeneity of the reef, act as reservoirs of zooxanthellae (Symbiodinium spp.), and also potentially counteract eutrophication via water filtering. Finally, dense populations of giant clams produce large quantities of calcium carbonate shell material that are eventually incorporated into the reef framework. Unfortunately, giant clams are under great pressure from overfishing and extirpations are likely to be detrimental to coral reefs. A greater understanding of the numerous contributions giant clams provide will reinforce the case for their conservation.