Maternal effects are non-genetic effects of the maternal phenotype or environment on the phenotype of offspring. Whilst maternal effects are now recognised as fundamentally important in terrestrial systems, they have received less recognition in the marine environment despite being remarkably common. The authors review the maternal effect literature and provide a simple framework for understanding maternal effects that increase offspring fitness (termed anticipatory maternal effects) and maternal effects that increase maternal fitness at the expense of offspring fitness (termed selfish maternal effects). The review then addresses various well-studied (offspring size effects, maternal care, oviposition effects) and poorly studied (manipulating offspring dispersal potential, toxicant resistance, sibling competition, mate choice) examples of maternal effects in the marine environment with a focus on marine invertebrates and fish. Offspring size effects are strong and pervasive in the marine environment but the sources and underlying causes of offspring size variation remain poorly understood. More generally, the authors suspect that changes in offspring phenotype are often adaptive maternal effects in response to environmental change. Maternal effects are of particular importance to marine systems because they not only form a link between the phenotypes of different generations, but the biphasic life cycle of most marine organisms suggests that maternal effects also link the phenotypes of populations.