Apex predators – such as pinnipeds, cetaceans, seabirds and sharks – are constrained by the sizes of prey they can consume and thus typically feed within a narrow range of trophic levels. Having co-evolved with their prey, they have influenced the behaviours, physiologies, morphologies and life-history strategies of the species they target. In contrast, humans can consume prey of any size from all trophic levels using methods that can rapidly deplete populations. On an ecological time scale, fisheries, like apex predators, can directly affect the abundance of other species by consuming or out-competing them; alternatively they can indirectly affect the abundance of non-targeted species by removing other predators. However, there is growing evidence that the effects of fisheries go well beyond those imposed by apex predators. Theory and recent observations confirm that the continued development and expansion of fisheries over the past half century has led to a decrease in the size and life spans of targeted species, with reproduction of fish occurring at earlier ages and at smaller sizes. In addition, high levels of fishing have altered the makeup of many ecosystems, depressing the average trophic level of heavily fished ecosystems and speeding up the rate of nutrient turnover within them. Inevitable consequences of fishing down the food web are increased ecosystem instability, unsustainable fisheries and an inability for the ecosystem to support healthy, abundant populations of apex predators.