ABSTRACT Costa Rica is a democratic exemplar, having maintained a critical mass of female legislators over two decades and approached gender parity in political offices; thus, it is a useful context for examining theories of change for gender representation. We use process-tracing to track the evolution of descriptive representation and examine when and how critical actors affect the legislative process in Costa Rica. We evaluate the extent to which critical mass representation has advanced substantive gender representation—the adoption of legislation favorable to women’s interests—through case studies of the 2020 law on street sexual harassment, and the 2022 law on violence against women in politics. We find that greater descriptive representation provides a pivot point for substantive representation, although critical actors in the form of key legislators and women’s organizations provide the necessary agency for legislative successes. The case study contributes to both theories and methods for understanding how descriptive representation and substantive representation interact in the democratic legislative process.