This article examines and explains the adoption of gender mainstreaming by the European Union (EU), and traces its implementation in five issue-areas of EU policy: Structural Funds, employment, development, competition, and science, research and development. The EU decision to adopt gender mainstreaming, as well as its variable implementation across issue-areas, can be explained in terms of three factors derived from social movement theory: the political opportunities offered by EU institutions in various issue-areas; the mobilizing structures, or European networks, established among the advocates of gender equality; and the efforts of such advocates to strategically frame the gender-mainstreaming mandate so as to ensure its acceptance by EU policy-makers.