Modelling is a diverse and complex area. It is also a controversial one. For more than two decades, researchers have developed and adapted models of widely differing kinds, ranging from econometric systems designed to forecast changes at the global market level to theories about individual decision processes. There has been considerable activity and enthusiasm, among research practitioners and marketing managers as well as marketing academics. On the other hand, some researchers and marketing men have reacted with either indifference or scepticism. Models have been condemned as too complicated, both conceptually and methodologically; as necessitating long and costly procedures of data collection and analysis; as being difficult to implement; and as irrelevant for practical decision making. These criticisms have stimulated protagonists of the modelling approach to pay more attention to building usable models and to finding ways of getting models used. Successful case histories have been reported and two are quoted later in the chapter. The purpose of this chapter is to draw attention to some major issues in the area.