This article presents a systematic review of the research methods used to study quality improvement (QI) effectiveness in health care organizations. The review relied on existing literature as well as emergent themes to identify types of QI programs (e.g., data/feedback, information technology, staff education) and quality outcomes (e.g., mortality, morbidity, unnecessary variation). Studies were separated into four categories according to the type of organization in which the QI program was introduced: (a) hospital, (b) nursing home, (c) physician group, and (d) other health care organization. Results of the review indicate that most QI effectiveness research is conducted in hospital settings, is focused on multiple QI interventions, and utilizes process measures as outcomes. The review also yielded substantial variation with respect to the study designs used to examine QI effectiveness. The article concludes with a critique of these designs and suggestions for ways future research could address these shortcomings.