As the human population has grown over recent decades, our profound impact on the entire planet has become starkly apparent. Increasing affluence has led to expectations of good-quality health care, full employment, and the ability to live in cities that are safe, affordable, and clean. Yet these advantages for some come at a cost to others and to the planet itself. Human activities have led to a warming climate, massive landscape conversion, pollution, depletion of natural resources (crucially, fresh water and soil), defaunation, and loss of biodiversity. Today, global human society stands at a decision point. Business-as-usual approaches are likely to lead to catastrophic changes to our planet and our health and well-being. What will it take for universal recognition of our perilous position, and how can we begin to make the often-difficult changes required to live in a more sustainable, cooperative, and compassionate way? In this special series, we call attention to these choices and explore some of the possible routes to a more sustainable future.