Primary forests are forests that are: (1) largely undisturbed by industrial-scale land uses and infrastructure such as logging, mining, and dams and roads; (2) the result of ecological and evolutionary processes including the full range of successional stages over time and with natural disturbance processes operating within historic bounds; (3) more likely to possess the full complement of their evolved, characteristic plant and animal species with few if any exotics; (4) dominated by a largely continuous tree canopy cover, and (5) have unpolluted soil and water. These forests are major strongholds for biodiversity and include the planet's most biodiverse terrestrial ecosystems. They provide essential climate change mitigation and adaptation, freshwater, and other ecosystem service benefits. They are also critically important for livelihoods and for cultural and spiritual reasons and are home for many Indigenous Peoples and local communities. Primary forests are unfortunately in rapid decline globally. Over centuries, Earth has lost about 35% of its preagricultural forest cover. Today, only about 32% (~ 1.3 billion hectares) of the world's forests are primary forest. We know that protected areas, indigenous, and community conservation are effective mechanisms for maintaining primary forests. National and international forest policies should prioritize the conservation of our planet's remaining primary forests.