This Article focuses on lessons learned from state practice in environmental justice (EJ) mapping and screening, and their relationship to the central issue of cumulative impacts—the reality that EJ communities typically suffer from a concentration of pollution sources and negative land uses as well as health and social vulnerabilities. These lessons are based on work in California and the development, use, and impact of the California Environmental Protection Agency’s CalEnviroScreen tool; the Article also examines the U.S. EPA’s EJSCREEN because of the ways that federal policies, tools, and data influence activities across all states. Five key lessons are discussed: (1) Addressing cumulative impacts is a core strategy for advancing EJ, and this is embodied in EJ mapping tool development; (2) Guiding principles for developing an EJ mapping tool can be articulated; (3) EJ mapping tools can help facilitate resource investment to promote health and sustainability in EJ communities; (4) Emerging EJ mapping efforts provide a useful, straightforward, and replicable model that state and local governments can emulate; and (5) Progress in advancing EJ at the state level, including mapping tool development, has come from the combined efforts of communities, academia, and government.