We synthesize the results of empirical studies of marine reserves to assess the potential benefits of protection for fish populations. Our meta-analyses demonstrate that the overall abundance of fishes inside reserves is, on average, 3.7 times higher than outside reserve boundaries. This enhancement is mainly a result of a significant increase in abundance of species that are the target of fisheries. Non-target species are equally abundant inside and outside reserves. Large-bodied species also respond more to protection, irrespective of their fishery status. Species within genera show great heterogeneity in their response to protection despite similarities in their life histories. Our study confirms that marine reserves benefit fish populations and highlights the need for monitoring prior to reserve establishment to provide more accurate, habitat-controlled studies of the effects of marine reserves on fish populations.