Our planet’s ability to provide an accommodating environment for humanity is being challenged by our own activi-ties. The environment—our life-support system—is changing rapidly from the stable Holocene state of the last 12,000 years, during which we developed agriculture, villages, cities, and contemporary civilizations, to an unknown future state of significantly different conditions. One way to address this challenge is to determine “safe boundaries” based on fundamental characteristics of our planet and to operate within them. By “boundary,” we mean a specific point related to a global-scale environmental process beyond which humanity should not go. Identifying our planet’s intrinsic, non-negotiable limits is not easy, but here we specify nine areas that are most in need of well-defined planetary boundaries, and we explain the steps needed to begin defining and living within them.