Drawing on an international literature review, two international workshops and primary qualitative research in Uganda this paper reviews experiences of mainstreaming HIV/AIDS in development sectors (such as education, health and agriculture) in developing countries. The extent to which HIV/AIDS mainstreaming strategies and associated challenges are similar to or different from those of mainstreaming gender in the health sector is also explored. The paper details the rationale for HIV/AIDS mainstreaming through illustrating the wide reaching effects of the pandemic. Despite the increasing interest in mainstreaming HIV/AIDS there is little clarity on what it actually means in theory or practice. This paper presents a working definition of HIV/AIDS mainstreaming. It is argued that all too often processes of 'mainstreaming' emerge as too narrow and reductionist to be effective. The paper then considers four key challenges for mainstreaming HIV/AIDS and explores how and to what extent they have also been faced in gender mainstreaming and what can be learnt from these experiences. These are: (1) the limited evidence base upon which to build mainstreaming strategies in different country contexts; (2) the role of donors in mainstreaming and implications for sustainability; (3) who should take responsibility for mainstreaming; and (4) how to develop capacity for mainstreaming. The conclusion argues for more joined up thinking and sustainable approaches to mainstreaming both HIV/AIDS and gender.