This article is a contribution to the debate on whether to mainstream gender, and how to mainstream gender, from the UK Gender and Development Network (GADN) Gender Mainstreaming Working Group. It draws on nine case studies of gender mainstreaming in the UK-based offices of international non-government organisations, and finds a complex but generally positive picture of progress. It concludes that the case for gender mainstreaming remains valid. Women’s projects on their own are limited in their ability to bring about fundamental change for women - complementary gender mainstreaming efforts are required to ensure that all development spending takes women’s rights and gender equality into account. Success in gender mainstreaming depends on the skills, resources, and influence of internal gender advocates, in combination with the effect of external influences on the enabling environment of the organisation. Gender mainstreaming is a long-term process which requires time, resources, skill, and persistence - but there is clear evidence of positive change.