Preventable harm, such as diagnostic and medication errors, threaten patient safety in ambulatory care settings. This article discusses the scientific, practice, policy, and patient/family milestones necessary to accelerate progress in reducing preventable harm among outpatients and advance ambulatory safety. The authors recommend numerous key milestones, including improving measurement methods, routine monitoring of safety for improvement and learning, leveraging patient engagement, and a national patient safety center to coordinate and lead ambulatory safety efforts.