Efforts to promote the completion of advance directives implicitly assume that completion rates of these documents, which help ensure care consistent with people's preferences in the event of incapacity, are undesirably low. However, data regarding completion of advance directives in the United States are inconsistent and of variable quality. We systematically reviewed studies published in the period 2011-16 to determine the proportion of US adults with a completed living will, health care power of attorney, or both. Among the 795,909 people in the 150 studies we analyzed, 36.7 percent had completed an advance directive, including 29.3 percent with living wills. These proportions were similar across the years reviewed. Similar proportions of patients with chronic illnesses (38.2 percent) and healthy adults (32.7 percent) had completed advance directives. The findings provide benchmarks for gauging future policies and practices designed to motivate completion of advance directives, particularly among those people most likely to benefit from having these documents on record.