Current population and development trends are leading to an acute deterioration of the environmental state of the oceans. One of the tools to manage the world’s seas more sustainably is marine protected areas(MPAs). MPAs are legally designated areas to protect and conserve valuable marine and coastal biodiversity and associated ecosystem services and cultural values. The recent pace of MPA designation and coverage has resulted in large increases in global MPA coverage, currently at 6.4%. However, most of this area concentrates in few huge offshore MPAs designated by few countries and lacks effective management. MPAs can be effective at preventing important local pressures such as overexploitation of marine resources or habitat destruction, but may be of little or no use to tackle some broad-scale marine pressures such as pollution or global warming. Although legal stringency is a broadly acknowledged factor for MPA environmental effectiveness, their ecological and socioeconomic effects are very context-specific and trade-offs among environmental features, between environmental and socioeconomic effects, and among socioeconomic stakeholders, are expected. Future marine conservation challenges include increasing demand for marine resources, global change, and scarce management budgets. In contrast, positive trends in MPA coverage, aquaculture production, and water quality make us slightly optimistic on the future of our oceans. Nevertheless, ultimate sustainability of the world’s seas will very much depend on global human population, consumption, and behavioral patterns.