<p>The role of litigation in politics, policy change, and advocacy remains insufficiently understood in part due to challenges in the availability, comparability, and transparency of legal data. This paper evaluates six foundational sources of U.S.-based federal litigation data - LexisNexis/NexisUni, Westlaw, the Federal Judicial Center Integrated Database, the Public Access to Court Electronic Records database, and two datasets from CourtListener, an open data source - with a focus on environmental law as an illustrative domain. We show that data source choice fundamentally shapes inferences about litigation trends, substantive focus, and the engaged players (litigants). We also show how data sources can be harmonized to make them more comparable, although harmonization may have some analytical cost. More broadly, we caution against relying on any single data stream and advocate for using multiple legal data sources to, at a minimum, illuminate features and omissions of legal data. Our findings highlight important methodological and theoretical implications for social scientists studying legal advocacy and interest group advocacy more broadly, including field-level advocacy dynamics, venue shopping, and patterns of advantage or disadvantage across legal conflict. While our focus is on the U.S., researchers across national contexts can improve how litigation is measured and understood by increasing the awareness of the strengths and limitations of legal data sources, which will in turn strengthen research on advocacy in the courts and the broader politics of policymaking.</p>