If the achievement of a truly gender sensitive parliament (GSP) is predicated on institutional change, then how might that change happen? In this chapter, I consider the promise and challenge of gender mainstreaming as a catalyst for parliamentary change. Gender mainstreaming is a critical pillar of GSP work because, by definition, it aims to normalise a gender lens across all of its “mainstream” work: representation, legislation and oversight (Palmieri, 2019). In this sense, gender mainstreaming is more than just a proxy for GSP—it is foundational to the whole transformation project. Problematically however gender mainstreaming has found itself at the intersection of policy and institutional change, with expectations that it can do both. These expectations—often unmet—ensure the continued importance of exploring gender mainstreaming in theory and practice across diverse contexts. I begin by reviewing gender mainstreaming as a catalyst for policy change, considering its drivers and mechanisms and their efficacy in producing gender sensitive government policy. Noting that this work identified, but struggled to operationalise, the role of institutions in enabling/hindering gender sensitive policy, I then look more specifically at the research that has applied gender mainstreaming to parliament, notably grounded in feminist institutionalism, and use this to articulate a conceptual framework.