The Joint Commission mandates systems for medication reconciliation. While best practices stem primarily from the inpatient setting, similar efforts in the ambulatory arena are largely descriptions of local improvement initiatives. This article provides a detailed summary of a medication reconciliation process that draws on 2 years of experience and a review of nearly 25,000 medication lists. The authors discuss errors discovered at the time of reconciliation and noted a rate of 31 changes per 100 medications reviewed. Broad staff engagement and careful attention to operational issues allowed for both a successful program and one that the authors believe is applicable to many ambulatory settings. The lead author, Dr. Saul Weingart, co-authored a past AHRQ WebM&M perspective that discussed organizational changes at Dana-Farber Cancer Institute following highly public medication errors.