Advances in patient safety in the past 2 decades have focused mainly on inpatient settings, whereas outpatient settings have been relatively overlooked. However, accumulated evidence leaves little justification to continue neglecting ambulatory safety. 1 A systematic review from 2015 estimated that safety incidents, such as those related to administrative and communication issues, missed or delayed diagnoses, and prescribing and medication management errors, occur in median of 2 to 3incidentsper100primarycarevisits. 2 Otherstudieshave estimated that 5% of US adult outpatients may have experienced a diagnostic error annually 3 and that a projected 4.5 million ambulatory care visits annually in the US may have been related to an adverse drug event. 4 Although errors in ambulatory settings are less likely to lead to immediate harm than errors in acute/inpatient care, their health consequences may be significant nonetheless (eg, from missed cancer diagnosis). Recognizing the need to address outpatient safety, reports from national and international groups have offered an impetus to measure ambulatory patient harm, prioritize nearterm goals, and create high-profile initiatives similar to those for inpatient settings. 1 Several barriers limit progress toward next steps. Care fragmentation and inadequate data systems make it difficult to map longitudinal patient experience in and across multiple settings (eg, primary/specialty care, patient home, diagnostic testing, community-pharmacy), hindering detection and assessment of safety problems. Consider a patient who, after visiting primary care and multiple specialty care settings, 2 laboratories, an imaging center, and an ambulatory surgery center, is diagnosed with lung cancer 1 year after initially presenting with fatigue and weight loss. Identifying this event and then determining sources of diagnostic delay in this patient's clinical course is a challenging task, even if medical records from each setting are easily accessible. Resources, infrastructure for measurement and analytics, and accreditation requirements for safety are less developed for ambulatory settings compared with inpatient settings. Additionally, patients can have complex presentations that include undifferentiated symptoms, multiple chronic conditions, and social needs that warrant attention in time-pressured, information-poor work environments.While the limitations of existing measurement methods have made it difficult to precisely quantify the overall prevalence of harm, evidence suggests that medication errors, diagnostic errors, and communication and coordination breakdowns are the most common causes of preventable harm among outpatients. The scope and consequences of harm include physical, psychological, and financial harms to patients, caregivers, health care workers, and society. A roadmap of milestones is needed to accelerate progress (eFigure in the Supplement).