Energy-critical elements (ECEs) are chemical and isotopic species that are required for emerging sustainable energy sources and that might encounter supply disruptions. An oft-cited example is the rare-earth element neodymium used in high-strength magnets, but elements other than rare earths, for example, helium, are also considered ECEs. The relationships among abundance, markets, and geopolitics that constrain supply are at least as complex as the electronic and nuclear attributes that make ECEs valuable. In an effort to ensure supply for renewable-energy technologies, science decision makers are formulating policies to mitigate supply risk, sometimes without full view of the complexity of important factors, such as unanticipated market responses to policy, society’s needs for these elements in the course of basic research, and a lack of substitutes for utterly unique physical properties. This article places ECEs in historical context, highlights relevant market factors, and reviews policy recommendations made by various studies and governments. Actions taken by the United States and other countries are also described. Although availability and scarcity are related, many ECEs are relatively common yet their supply is at risk. Sustainable development requires informed action and cooperation between governments, industries, and researchers.