Twenty years after the passing of Resolution 1325, the participation of women as military personnel in peacekeeping operations remains limited. Women currently comprise just under five per cent of military personnel in UN peacekeeping missions, and the UN consistently calls for more. We contend the low numbers of female military personnel in peacekeeping reflects a lack of gender mainstreaming in national militaries globally. This article introduces the problem of sidestreaming, the practice, deliberate or unintentional, of sidelining women and relegating them to specialised spaces in international peace and security while attempting gender mainstreaming or increased gender integration. Drawing on empirical evidence from national militaries we show how and where sidestreaming occurs with the result being that women remain clustered in gendered and low-status spaces in national militaries and in specialised spaces in peacekeeping operations. This has a negative effect on retention and recruitment contributing to the low numbers of female peacekeepers in UN peace operations. We conclude that gender mainstreaming in its fullest sense will require military reform that decouples violence and combat skills from masculinity and inclusive research strategies that engage men as well as women.