During the past decade, there has been renewed commitment to programmes for helminth control, and several international initiatives have been launched. Scientific evaluation of large-scale interventions to reduce the incidence of infection and associated morbidity and mortality is vital, not only to demonstrate health benefits, but also to assess cost-effectiveness to show that monies from donor agencies have been used wisely. Using the example of schistosomiasis, this article outlines a framework for the evaluation of the impact of national control programmes, highlighting the important epidemiological and practical issues that must be addressed.