Planning and evaluation of parasitic disease control is complicated by the many interacting factors that jointly determine the epidemiological trends under different control strategies. The Onchocerciasis Control Programme (OCP) of the World Health Organization in West Africa has recognized this problem and uses epidemiological modelling as on aid to addressing control questions. Dik Habbema, Edoh Soumbey Alley, Anton Plaisier, Gerrit van Oortmorssen and Hans Remme describe the organization of modelling in the OCP and summarize the most important achievements thus far. The experience with applied modelling in OCP is of considerable interest for other disease control programmes.