Diagnostic errors are a common cause of patient harm in ambulatory care. Although such errors have often been ascribed to cognitive biases, this study highlights physicians' concerns that health system structures and communication are major drivers of delayed and missed diagnoses. Focus group discussions involving 25 outpatient physicians—primarily from internal and family medicine—identified multiple potential sources of diagnostic errors, including insufficient information availability, disjointed workflows, and poor communication among providers and with patients. This study underscores many overlapping issues that will need to be addressed to meaningfully enhance diagnostic accuracy. In a recent AHRQ WebM&M interview, Dr. Urmimala Sarkar, the lead author of this study, discussed patient safety in the ambulatory setting.