Since the very first Earth Day more than four decades ago, the environmental movement has tackled a wide range of problems, including air pollution, contaminated water, deforestation, biodiversity loss, and more. But this Earth Day, I propose that there are two fundamental issues the movement must address over the coming decade if it is ever to defuse the tension between development and the environment. In fact, these two issues underlie many, if not most, of the world’s environmental challenges.