We review quantitative studies of the cephalopod diet of seabirds, with details of all species forming more than 5% by numbers or mass of seabird diets. Although squid are widespread as food for marine birds, only for some albatross and petrel species are they consistently as important as fish or crustaceans. Nevertheless, several penguins, auks and terns take significant quantities of squid at some sites and seasons. Although most of the detailed studies have been on temperate and polar seabirds in the southern hemisphere, squid may play a key role in the diet of many tropical seabirds. Generally, squid may be more important to many marine birds outside the breeding season than hitherto documented. Many species and families of squid are eaten by seabirds but Ommastrephidae, Onychoteuthidae, Histioteuthidae and Gonatidae probably make the greatest contributions. Evidence for size and species selectivity, except as constrained by the size and habits of seabirds, is weak. How seabirds catch squid is reviewed, covering the topics of scavenging and live capture and association with cetaceans. In general, seabirds have much smaller known and potential impact on squid stocks than do marine mammals. However, seabirds are probably the best samplers of squid populations currently available and can provide valuable data for the identification of potential, and management of existing, commercial fisheries. Future research needs, especially for studying the dynamics of squid-seabird interactions, are reviewed.