In sexually reproducing organisms, conflicts of interest among family members are inevitable. The intensity of these conflicts depends upon the opportunities for parents and offspring to interact and the level of promiscuity. Despite the acknowledged role of family conflict in the evolutionary ecology of terrestrial organisms, its influence in the marine realm has largely been ignored. Nevertheless, marine organisms exhibit a wide range of reproductive and developmental modes through which sexual, sibling, and parent-offspring conflicts can manifest. Moreover, the existence of multiple mating in these species increases the likelihood, as well as the degree, of these conflicts. Consequently, many puzzling aspects of evolution in the sea, from life-history variation to diversification, could be clarified through the lens of conflict theory.