Fish and shellfish supply the human diet with not only complex nutrients including the ω-3 fatty acids, but also highly toxic chemicals including methylmercury. The dietary essential fatty acids are linoleic and α-linolenic acid. Two ω-3 fatty acids with longer carbon chains, eicosahexaenoic acid (EPA) and docosahexaenoic acid (DHA), can be synthesized in humans from α-linolenic precursors. Though not required in the diet per se, EPA and DHA have important roles in metabolism. The almost exclusive source of preformed dietary DHA is fish and shellfish. These foods are also an important source of EPA. In marked contrast to the benefits of fish and shellfish as sources of preformed ω-3 fatty acids, fish and shellfish are almost exclusively the dietary source of methylmercury. Fortunately, these chemicals are not uniformly distributed across many species of fish and shellfish. The purpose of this article is to provide information on the comparative distribution of these chemicals and nutrients to help groups formulating dietary recommendations.