In this article, the systematic development of an Internet HIV-prevention intervention, based on the Intervention Mapping (IM) protocol, is described. As such, the present article meets the recent calls for more detailed descriptions of the development of health promotion programs. In doing so, the different steps of IM are introduced and applied to the promotion of HIV-testing among sexually active Men who have Sex with Men. Particular attention is given to the Internet setting of the program, which is called "Queermasters". The additional value of IM to the development of Internet health promotion programs and the advantages of using the Internet for health promotion purposes are considered.