Since 1990 the Assistance Publique--Hôpitaux de Paris (AP-HP) developed a policy of health care assessment in agreement with the implications of the law of July 31, 1991. The department of Evaluation of health care, in close collaboration with the local Committees which operate in most of the hospitals of AP-HP, performed during the last three years about fifty studies dealing with two main topics: professional procedures and quality of care. Through three specific selected studies, the authors emphasize the requirements of an assessment policy to efficiently contribute to the improvement of quality of care.