The conditions for life on Earth are changing profoundly. The climate crisis is causing frequent forest fires, heat waves, floods and extreme drought. Biodiversity is rapidly declining, environmental pollution has reached every corner of Earth, nitrogen is accumulating in nature, deforestation and erosion are continuing at an alarming rate, and freshwater sources are drying up. - If these global environmental changes continue unabated, billions of people worldwide may face serious health risks by the end of this century. Many millions may die every year in that event, and many more could suffer from heat stress, infectious diseases, malnutrition, flooding and mental disorders. Some human communities will find their very existence threatened as their habitat is flooded or scorched by heat or drought. According to the World Health Organization, climate change is ‘the single biggest health threat facing humanity’ in the 21st century.- Yet scientists are only beginning to study the impact of global environmental change on human health, and how this impact can be abated. ‘Planetary Health’ was launched as a new field in 2015 and has since grown rapidly. It is a broad field, going beyond the local environmental threats studied in the past and focusing on changes on a planetary scale. This report presents the views of the Royal Netherlands Academy of Arts and Sciences (KNAW) on this emerging field. It identifies the main knowledge gaps and proposes a research agenda, assembled in consultation with a wide range of experts. Planetary Health as a scientific field is based on the understanding that all life on Earth is interconnected, and that safeguarding the health and survival of all species is an important moral imperative. For practical reasons, however, this report focuses mainly on human health, to which most research efforts have so far been devoted.