This article integrates the comparative literature on gender quotas with the existing body of research on women's substantive representation. Quota laws, which bring greater numbers of women into parliaments, are frequently assumed to improve women's substantive representation. We use the Argentine case, where a law mandating a 30% gender quota was adopted in 1991, to show that quotas can affect substantive representation in contradictory and unintended ways. To do so, we disaggregate women's substantive representation into two distinct concepts: substantive representation as process, where women change the legislative agenda, and substantive representation as outcome, where female legislators succeed in passing women's rights laws in the Argentine Congress. We argue that quota laws complicate both aspects of substantive representation. Quotas generate mandates for female legislators to represent women's interests, while also reinforcing negative stereotypes about women's capacities as politicians. Our case combines data from bill introduction and legislative success from 1989 to 2007 with data from 54 interviews conducted in 2005 and 2006. We use this evidence to demonstrate that representation depends on the institutional environment, which is itself shaped by quotas. Institutions and norms simultaneously facilitate and obstruct women's substantive representation.