Environmental data play an increasingly important role in the politics of environmental justice (EJ) in the United States. This article presents the results of a case study focused on how emissions and health data have been used by EJ advocates seeking to influence climate change legislation in California over the past two decades. These results highlight the importance of publicly accessible environmental databases. This study describes how California's Environmental Protection Agency, which was assigned responsibility for the health and welfare of EJ communities, sought to enhance the utility of the available emissions data by requiring the development of a new state analytical tool called CalEnviroScreen. This tool and methodology, which continues to be expanded and maintained by the state, were used by EJ justice advocates to influence decisions by state legislators as they developed climate-related emission reduction strategies, including a market-based cap-and-trade program. Case study implications include the importance of having “a seat at the table,” “access” to public data, and the development of analytical tools that enable EJ representatives to participate in the policymaking process. Final observations include the results of recent air quality audits that suggest that reductions in pollutants in disproportionately impacted communities in California have not yet been achieved, raising questions about the limits of data-driven approaches to achieving EJ that are not backed by enforceable pollution reduction requirements.