Much of the recent progress in the study of eco-evolutionary dynamics has occurred in fish models. In this review, I focus specifically on the mechanisms by which fish evolution affects ecological processes, highlighting outstanding questions and new frontiers. Fish evolution can alter the ecology of individuals by changing traits that underlie resource use or interactions with other species. Fish eco-evolutionary studies have focused on the ecological consequences of two kinds of effect traits (trophic traits and nutrient recycling traits), but questions remain about the relative influences of genetic and environmental factors. Other fish traits might also have large ecological effects, but we lack frameworks for identifying them, and it remains difficult to link the traits to their ecological consequences. The effect of fish evolution also depends on changes in fish populations, which might vary because of genetic or by environmental factors. Such changes can have dramatic impacts on the expression of effect traits and represent an important, yet largely unexplored conduit linking evolutionary change to community and ecosystem effects. At the level of the community, fish evolution has been shown to induce different kinds of trophic cascades, although it is not clear how patterns observed in simple eco-evolutionary experiments with simple communities are a realistic representation of what happens in complex natural communities. Although fish evolution has also been shown to cause changes in ecosystem function, explaining the mechanisms is difficult because changes in ecosystem function are influenced by changes in all other levels of organization as well as by abiotic heterogeneity. Improving our understanding of the effects of fish evolution on communities and ecosystems will therefore require sophisticated experimental or statistical tools to link causes and effects, and also to complement eco-evolutionary experiments with more studies in natural systems. Continuing to improve our understanding of why fish evolution causes ecological effects will be critical as we expand the scope of studies to include feedbacks, more diverse fish models, and particularly as we apply our understanding of eco-evolutionary dynamics to fish conservation in impacted landscapes.