In 2004, AHRQ launched a collection of evidence reports, Closing the Quality Gap: A Critical Analysis of Quality Improvement Strategies, to bring data to bear on quality improvement opportunities. These reports summarized the evidence on quality improvement strategies related to chronic conditions, practice areas, and cross-cutting priorities.The United States devotes significant resources for the provision of health care, yet quality is often elusive or lacking. In 2004, the Agency for Healthcare Research and Quality launched a collection of evidence reports to bring data to bear on quality improvement (QI) opportunities. This new series, Closing the Quality Gap: Revisiting the State of the Science, consists of eight reports that continue the focus on improving the quality of health care through critical assessment of relevant evidence for selected settings, interventions, and clinical conditions. This report is an introduction to the Executive Summaries of the eight reports in the series and summarizes elements across the series for readers.