Ecologists have known for some time that predators can have significant effects on terrestrial and aquatic prey populations. Culling is widely practiced as a means to limit predation on livestock and game. Changes in species' distributions and abundance illustrate that culling programs can be very effective at reducing predator density. Culling has also been used to reduce marine mammal populations in many parts of the world. Coastal pinniped species have usually been the target of such programs, but dolphins and large cetaceans have also been culled. The extent of marine mammal population reduction and the response of targeted prey populations to culls have rarely been evaluated. There are several conclusions that can be drawn from experimental studies in terrestrial systems and more model-based approaches in aquatic systems. First, predator removal can increase productivity and population size of target prey populations, but not always. Second, these studies typically have involved large proportional reduction (>50%) in predator populations, presumably to increase effect size and the statistical power to detect a significant effect. Third, the effects of culling are typically dependent on continued control, and in the absence of control the benefits rapidly disappear. This underscores the need for predator removal to be a long-term management strategy. Fourth, at least in the case of marine mammals, few studies have clearly articulated measurable objectives for prey population recovery or increase and most have not evaluated the success of the control program with respect to those objectives. Fifth, culling predators often has non-intuitive and unintended consequences for both target and other predator and prey species. Despite their prevalence, the effectiveness, efficiency and the benefit: cost ratio of culling, programs have been poorly studied.