Mass-litigation episodes involving claims alleging personal or financial injury or property damage are a familiar feature of the U.S. civil-legal landscape. Building upon seminal work by Galanter (1990) and Hensler & Peterson (1993), who view “case congregations” or “litigations” as interesting units of observation, the authors propose a conceptual framework for analyzing the roles of social, institutional, economic and legal factors that affect or are affected by mass litigation. The framework has three major parts: (1) the “core” of the litigation, comprising defendant behavior, injuries, and litigation activity; (2) exogenous influences on developments within the core; and (3) endogenous outcomes of the litigation activity within the core. Influences and outcomes fall into five categories, namely (1) legal doctrine and processes, (2) other litigation, (3) regulation, (4) media reports and (5) market factors. Examples from several mass-litigation episodes illustrate the concepts and suggest their relevance for understanding the world of mass litigation.