Very few examples of theory-driven and systematically developed weight gain prevention interventions for adults have been described in the literature. The present paper systematically describes the development, implementation and evaluation framework of a weight gain prevention programme directed at young adults at the worksite, namely the NHF-NRG In Balance-project. It not only can be used as a guide to systematically develop weight gain prevention interventions, but also gives an overview of the current theoretical and empirical knowledge-base in the field of obesity prevention. The outline of the paper follows the Intervention Mapping protocol, which includes a systematic inventory of important health issues, their risk behaviours and determinants of these risk behaviours, and specification of the proximal objectives of the programme directed at both energy intake and energy expenditure. The objectives are translated into behaviour change methods and strategies, which are combined in a stepwise intervention programme, and used for a detailed evaluation plan (process and effect evaluation). The NHF-NRG In Balance-project combines mass media and individually tailored communications with worksite environmental changes to raise awareness, to motivate and to enable energy balance behaviour changes. A quasi-experimental pre-test-multiple post-test control group design was applied in 12 worksites (>500 employees). © 2006 The Authors.