This paper, produced by the Gender and Development Network (GADN) Gender Mainstreaming Working Group, explores the concept and practicalities of gender mainstreaming. It draws on learning from staff with responsibility for gender mainstreaming in nine UK based international Non Governmental Organisations (INGOs) and their Southern based partner organisations; wider discussions with GADN members and women’s rights activists; as well as the personal experiences and reflections of the authors. It elaborates a Theory of Change setting out the component parts of gender mainstreaming, how these relate to each other, and how they collectively contribute towards the wider goal of gender equality and women’s and girls’ rights