What interventions are effective in influencing prescribing by general practitioners (GPs)? Two studies were conducted - a matched comparison survey of prescribing change in 26 practices forming a GP Commissioning Group (which piloted new ways to modify GP prescribing) and a longitudinal survey of 145 prescribing advisers (who were responsible for administering prescribing incentive schemes in their area). The effectiveness of differing financial incentives in influencing prescribing has already been described. This thesis supports the notion that non-profit making financial incentives, such as generating prescribing savings to be re-invested in other aspects of health care for the local community, may also be an effective motivator of prescribing change. GPs considered educational interventions to be the most effective approach but if prescribing were to change, interventions needed to tap into their professional values such as autonomy or the sense of belonging to a peer group. Interventions to change prescribing may be better targeted at individual GPs rather than the GP partnership as a whole since evidence was found that GPs tend to behave as individuals when considering prescribing change but collectively as partnerships in their approach to managerial issues. Since the formation of Primary Care Organisations (PCOs) in 1999, the predominant influence on prescribing has shifted away from emphasis on cost control toward attempts to improve prescribing quality. The effectiveness of interventions to improve prescribing quality is harder to demonstrate and remains largely unknown.