There are now a number of HIV/AIDS prevention programmes for migrant and ethnic minority communities throughout the world, both 'top down' programmes organised, for example, by governments and large NGOs, and 'bottom up' programmes, organised by migrant groups themselves. Evaluation of such programmes, however, is in most cases sorely lacking. The Swiss 'Migrants Project' is, to the authors' knowledge, the only such programme to have been systematically accompanied by evaluation throughout. This paper describes three phases of evaluation of the Migrants Project (exploratory studies, process, and outcome evaluations). The evaluations have highlighted the need for culturally and linguistically appropriate prevention efforts which use already-existing community structures, as well as the need to identify and train people from within communities to carry out local prevention efforts. Outcome evaluation has shown that: a government sponsored HIV/AIDS prevention programme can meet with acceptance by migrant communities; considerable engagement in prevention activities can be mobilised; and AIDS prevention among such communities can be effective. Such efforts can create levels of sensitivity to HIV issues and of protective behaviour that are equal to those of the host country population. The strategy adopted by the programme is thus supported. Key elements are to avoid potential for stigmatising by: (1) placing HIV/AIDS prevention efforts for migrant populations within an overall national HIV/AIDS prevention strategy; (2) informing and sensitising general populations within migrant communities before initiating more targeted prevention with migrant IDUs, MSM, and CSWs; (3) encouraging, facilitating and guiding health promotion efforts which emerge from within migrant communities themselves.