This review concerns the parasitic marine copepod Lernaeocera branchialis (L., 1767) and provides an overview of current knowledge concerning its biology and host-parasite interactions. The large size and distinctive appearance of the metamorphosed adult female stage, coupled with the wide exploitation and commercial importance of its final gadoid hosts, means that this species has long been recognised in the scientific literature. The fact that the Atlantic cod, Gadus morhua L., is one of its key host species, and has itself had a major impact on the social and economic development of many countries bordering the North Atlantic for more than 10 centuries is also a factor in its widespread recognition. L. branchialis is recognised as a pathogen that could have major effects on the aquaculture industry and with gadoid (especially cod) farming expanding in several North Atlantic countries, there is considerable potential for this parasite to become a serious problem for commercial mariculture. The main subject areas covered are the parasite's taxonomy; the life history of the parasite including its life cycle, reproduction and host associations; parasite physiology; parasite seasonality and distribution; and the pathogenic effects of the parasite on its host.