Reported rates of referral by individual general practitioners to hospitals range from less than 1% of all consultations to more than 20%. Research on variations in rates of referral by general practitioners in the UK is reviewed here. Studies have largely failed to account for variation either in terms of differences in the characteristics of patients or differences in the doctors and their practices. It is argued that this failure arises because most studies do not distinguish between different types of referral or reasons for making a referral. In order to begin to explain variations it is necessary to identify the stages in the complex process of decision making. A theoretical model of the referral decision is advanced, which is intended to provide a framework for further research on the referral process.