Popular quality improvement tools such as Six Sigma (SS) claim to provide health care managers the opportunity to improve health care quality on the basis of sound methodology and data. However, it is unclear whether this quality improvement tool is being used correctly and improves health care quality. The authors conducted a comprehensive literature review to assess the correct use and implementation of SS and the empirical evidence demonstrating the relationship between SS and improved quality of care in health care organizations. The authors identified 310 articles on SS published in the last 15 years. However, only 55 were empirical peer-reviewed articles, 16 of which reported the correct use of SS. Only 7 of these articles included statistical analyses to test for significant changes in quality of care, and only 16 calculated defects per million opportunities or sigma level. This review demonstrates that there are significant gaps in the Six Sigma health care quality improvement literature and very weak evidence that Six Sigma is being used correctly to improve health care quality.