The rate of cesarean delivery in the United States increased dramatically over the past two decades, from 20.7 percent in 1996 to 32.8 percent in 2010. Part of the reason for the increase is a decline in the rate of vaginal birth after cesarean (VBAC). Although the dictum "once a cesarean, always a cesarean" guided clinical practice for a good part of the 20th century, a 1980 National Institutes of Health (NIH) Consensus Development Conference Panel recognized trial of labor (TOL) after prior cesarean as a viable option for certain low-risk women.3 An increase in VBAC ensued; by 1996, more than 28 percent of women with a prior cesarean delivered vaginally. However, a number of medical and nonmedical factors, including reports in the 1990s of an increased risk of maternal complications with TOL compared with elective repeat cesarean, pushed the pendulum in the opposite direction. The percentage of women with a previous cesarean delivering vaginally fell from a peak of 28 percent in 1996 to 8.5 percent in 2007. In 2010, NIH again convened a Consensus Development Conference Panel to evaluate the growing body of evidence on the clinical risks and benefits of TOL after cesarean. In preparation for the 2010 conference, the Agency for Healthcare Research and Quality (AHRQ) commissioned the Oregon Evidence-based Practice Center (EPC) to conduct a review of the evidence on a number of emerging issues related to VBAC, which was released as AHRQ Evidence Report/Technology Assessment No. 191. The evidence review addressed the following six Key Questions. 1. What are the rates and patterns of utilization of trial of labor after prior cesarean, vaginal birth after cesarean, and repeat cesarean delivery in the United States? 2. What are the nonmedical factors (e.g., provider type, hospital type) that influence the patterns and utilization of trial of labor after prior cesarean? 3. Among women who attempt a trial of labor after prior cesarean, what are the vaginal delivery rate and the factors that influence it? 4. What are the short- and long-term benefits and harms to the mother of attempting trial of labor after prior cesarean compared with elective repeat cesarean delivery, and what factors influence benefits and harms? 5. What are the short- and long-term benefits and harms to the baby of maternal attempt at trial of labor after prior cesarean compared with elective repeat cesarean delivery, and what factors influence benefits and harms? 6. What are the critical gaps in the evidence for decisionmaking, and what are priority investigations needed to address these gaps?