The planetary phase of history has begun, but the future shape of global society remains profoundly uncertain. Though perhaps improbable, a shift toward a planetary civilization of enriched lives, human solidarity, and environmental sustainability is still possible. This treatise examines the historic roots of this fateful crossroads, analyzes alternative scenarios that can emerge from contemporary forces and contradictions, and points to strategies and choices for advancing a Great Transition. It synthesizes the insights of the Global Scenario Group, convened in 1995 by the Tellus Institute and Stockholm Environment Institute to explore the requirements for a sustainable and desirable future.