This paper is concerned with the R.B.L.-Forecast Motorists Diary Panel and sets out to illustrate how Amoco, a relative newcomer to the U.K. petrol market, has made effective use of the panel data in its development programme. The first part of the paper discusses the various technical parameters of the panel including the panel's representation and 'pick-up' and the validity of the data. It shows how experiments over the years have led to refinements in recruitment and panel maintenance techniques which have minimised initial 'drop-out' and maximised continuity. The second part of the paper discusses ways in which Amoco has used the data as a management tool in developing and evaluating its marketing strategy. Four recent applications are discussed.