The central purpose of this article is to review the major approaches to evaluation of health care quality in terms of its use of different data sources. The areas of application, advantages, and disadvantages of seven data sources are presented, including direct observation, clinical records, record abstracts, clinical and administrative staff survey, patient survey, significant others survey, and population survey. Then the selection of data sources is discussed as this process is influenced by the answers to seven critical questions concerning the context of the quality assurance effort.