Patient safety and quality improvement research is a relatively young field, and controversy exists about the most basic aspects of conducting studies in the field. While some have argued that patient safety research must be held to the same standards as other clinical trials, others have countered that challenges unique to patient safety research require a new paradigm for conducting and evaluating studies in this area. This four-part series explores all aspects of quality improvement research, including articles on conceptualizing and developing interventions, study designs, measurement of safety and quality, and the use of mixed methods studies. An accompanying editorial by David Bates, one of the pioneers of patient safety research, highlights the controversies that inspired publication of this series.