This article analyses the implementation of gender mainstreaming in the European Parliament and aims at deciphering the role of its committees and political groups in advancing or hindering the integration of gender perspectives. The article engages with feminist institutionalism and micro-political approaches, and is based on interview and documentary data. It examines how formal and informal institutions and micro-political strategies within committees and political groups affect the abilities of this representative European Union institution to ensure a gender perspective is present in European Union policies. We suggest that although the Committee on Women’s Rights and Gender Equality (the gender-focused parliamentary body) oversees gender mainstreaming, committees and political groups, as the core actors of European Parliament policymaking, are the gatekeepers that determine the outcomes. Our findings advance understandings of the limits of gender mainstreaming in European Union policymaking and shed light on the specific challenges of gender mainstreaming and broader gender equality change in parliaments.