OBJECTIVE: To lay the foundation for an explicit review and dialogue concerning the criteria that should be used to evaluate qualitative health services research. Clear criteria are critical for the discipline because they provide a benchmark against which research can be assessed. DATA SOURCES: Existing literature in the social sciences and health services research, particularly in primary care and medicine. PRINCIPAL FINDING: Traditional criteria for evaluating qualitative research are rooted in the philosophical perspective (positivism) most closely associated with quantitative research and methods. As a result, qualitative research and methods may not be used as frequently as they can be and research results generated from qualitative studies may not be disseminated as widely as possible. However, alternative criteria for evaluating qualitative research have been proposed that reflect a different philosophical perspective (post-positivism). Moreover, these criteria are tailored to the unique purposes for which qualitative research is used and the research designs traditionally employed. While criteria based on these two different philosophical perspectives have much in common, some important differences exist. CONCLUSION: The field of health services research must engage in a collective, "qualitative" process to determine which criteria to adopt (positivist or post-positivist), or whether some combination of the two is most appropriate. Greater clarity about the criteria used to evaluate qualitative research will strengthen the discipline by fostering a more appropriate and improved use of qualitative methods, a greater willingness to fund and publish "good" qualitative research, and the development of more informed consumers of qualitative research results.