In August 2006, Portugal approved a new quota law, called the parity law. According to this, all candidate lists presented for local, parliamentary, and European elections must guarantee a minimum representation of 33 per cent for each sex. This article analyses the proximate causes that led to the adoption of gender quotas by the Portuguese Parliament. The simple answer is that the law's passage was a direct consequence of a draft piece of legislation presented by the Socialist Party (PS), which enjoyed a majority. However, the reasons that led the PS to push through a quota law remain unclear. Using open-ended interviews with key women deputies from all the main Portuguese political parties, and national public opinion data, among other sources, the role of four actors/factors that were involved in the law's adoption are critically examined: notably, civil society actors, state actors, international and transnational actors, and the Portuguese political context.