The term "environmental problem" exposes a fundamental misconception: Disruptions of Earth's ecosystems are at their root a human behavior problem. Psychology is a potent tool for understanding the external and internal drivers of human behavior that lead to unsustainable living. Psychologists already contribute to individual-level behavior-change campaigns in the service of sustainability, but attention is turning toward understanding and facilitating the role of individuals in collective and collaborative actions that will modify the environmentally damaging systems in which humans are embedded. Especially crucial in moving toward long-term human and environmental well-being are transformational individuals who step outside of the norm, embrace ecological principles, and inspire collective action. Particularly in developed countries, fostering legions of sustainability leaders rests upon a fundamental renewal of humans' connection to the natural world.