RIA methodology has evolved over the last 30 years and has moved through several stages. At the same time there have been new directions in planning theory which have in turn influenced planning policy. Shifts have taken place in government policy towards retail development with changes in attitudes towards new forms of retailing, particularly in out-of-centre locations. A key issue in retail planning is whether major shopping developments have an unacceptable impact on existing town centres. In the mid 1990s this question has achieved high political profile and has become more significant because of growing public concern about the cycle of decline perceived in many town and city centres. Approaches to assessing retail impact have changed considerably over recent decades because of technical advances in planners' understanding of the retail system and through learning from past experience on the effects of new retail developments. But at the same time there has been a realisation that assessing the impact of a new shopping development is not simple; it is concerned with outcomes which cannot easily be predicted or quantified. Human behaviour and the retail system are too complex for retail impact assessment to be treated as a mechanistic exercise.