The ocean covers nearly three-fourths of the globe and is home to almost a quarter of the world's known species—with many more yet to be discovered. It plays an essential role in sustaining life by regulating global chemistry and climate and providing sustenance for billions of people. But human activities are increasingly threatening the ocean's health. Many experts and island communities now see the creation of marine reserves as a key tool for rebuilding the abundance and diversity of species while strengthening ocean resilience to climate change. Studies show that the ecological benefits of reserves extend well beyond their boundaries. Because of what is known as the 'spillover effect,' thriving populations of fish within closed areas, such as reserves, tend to move into nearby waters. Marine reserves yield the greatest conservation benefits when they are large, highly protected, isolated, well-enforced, and long-standing.