This paper describes the development of a Dutch online programme called "Long Live Love+" focusing on positive, coercion-free relationships, contraception use, and the prevention of STIs, using the Intervention Mapping (IM) approach. All six steps of the approach were followed. Step 1 confirmed the need for a sexual health programme targeting young people aged 15 and over enrolled in higher level secondary education. Step 2 resulted in the production of a series of matrices-of-changes, including detailed programme objectives at the behavioural and the psycho-social level. Step 3 involved the selection of relevant methods and applications. Step 4 consisted of programme development, resulting in a sexual health programme with online and offline components, and including interactive exercises. Step 5 focused on adoption and implementation and included the production of a detailed teacher manual. Step 6 involved detailed planning for the process and effect evaluation and included interviews with teachers and focus group discussions with students to evaluate their experiences of the programme. The inclusion of a linkage group--and especially the inclusion of teachers in the development of the programme--turned out to be essential in terms of developing a programme in line with their context and needs.