This chapter discusses the origins of gender mainstreaming as an institutional gender equality strategy and its contested definitions across states, international organisations and advocacy networks. It explores gendered power in policy and bureaucratic structures and how feminist International Relations scholars interpret the meaning of mainstreaming politics within particular organisations. The chapter considers the voice of women, women's movements and feminist advocates in gender mainstreaming. Feminist analyses give us crucial insights into the limitations of mainstreaming and its potential as a strategy for political transformation when harnessed by feminist advocates and deployed in specific institutions and local contexts. In some international organisations, such as the European Union and the World Bank, gender mainstreaming has conformed to a technocratic model where bureaucrats are the main actors relatively disconnected from women's activism in civil society. International organisations are often places of masculine dominance and bureaucratic myopia with limited democratic accountability to broader publics.