While the United Nations Sustainable Development Goals direct all countries to reduce environmental impacts, governments and businesses continue to promise and prioritize economic growth. Degrowth theory draws on the science of thermodynamics to illuminate systemic impacts of this growth: societal metabolisms transform material and energy into goods and services in processes that convert low entropy stocks of resources into high entropy waste. As accelerating metabolisms lead to more and more anthropogenic entropy, with outcomes ranging from imbalanced nutrient and hydrological cycles to climate break-down, a growing network of scholars and activists work toward degrowth objectives: to reduce global energy and resource use, curb obsessions with growth, and reorient societies around equitable wellbeing and resilience.