GP fundholding was the most radical element in the package of reforms introduced into the British National Health Service in April 1991. Despite initial hostility from the British Medical Association, this scheme has become increasingly popular with GPs, such that it now covers one-third of the UK population. GP fundholding has generated considerable interest internationally and the British Government has hailed It as a great success. When the organizational changes were first implemented formal evaluation was not encouraged by the government. Nevertheless, some hearth services researchers have carried out evaluative studies on the impact of GP fundholding. This paper discusses the problems faced by those attempting to evaluate hearth care reforms, using the studies on fundholding to illustrate the difficulties. A summary of the main findings from these studies reveals extensive gaps in current knowledge about the impact of the scheme. Claims that the introduction of GP fundholding has resulted in improvements in efficiency, responsiveness and quality of care are in general not supported by the evidence.