Most (84%) of the warming due anthropogenic climate change has been transferred to the oceans. This chapter outlines the causes and consequences of climate change and summarise future projections for ocean temperature rise, coral bleaching events and ocean acidification, and the associated uncertainties. This review largely focuses on marine ecosystems, as three quarters of capture fisheries landings come from the seas. However, it also presents key issues and examples from freshwater fisheries, as these fisheries provide important livelihoods and fish protein for some of the world’s poorest people. While the physical and biological effects of climate change are increasingly well understood, particularly for well-studied temperate shelf ecosystems, relatively little is known of the likely impacts for ecosystems elsewhere and their associated fisheries. Overall, on balance, climate change appears to have impacts on fish ecology and fisheries, but the strength and direction (positive or negative) of the effects vary from place to place. The social and economic effects are less clear; however it is likely that the economies of countries with the lowest levels of adaptive capacity will be most vulnerable to the effects of climate change on capture fisheries and less able to anticipate and capitalise on any advantages of climate impacts. Despite the uncertainty surrounding the direction and degree of the impact of climate change on marine and freshwater ecosystems, and the associated fisheries and fishing communities, the options for policy makers are relatively clear. Policy makers can respond by pursuing mitigation strategies (reducing CO² emissions), building socio-ecological resilience and capacity to enable fishing communities to cope with and adapt to the opportunities, challenges and potential dangers presented by climate change, and by integrating the management of natural resource sectors in a portfolio approach.