The authors discuss the development of a flexible Attitude -to - Behaviour model, originally developed to explore viable product developments in the communications market. The model has been applied to pharmaceutical marketing research, using patient types derived by cluster analysis to simulate the range of different patients treated by physicians. The modelling process has three distinct phases outlined in detail in the body of the paper: 1. The Building Phase: where the psychological value of each product/service examined is computed as a weighted linear sum of components 2. The Validation Phase: to assess the fit of the specified model 3. The Prediction Phase: when simulation experiments (including stochastic and marketing mix variables) are conducted to estimate likely purchase/prescribing behaviour. These phases may be iterative.