Increasing use of advanced medical imaging is often cited as a key driver of cost growth in medical spending. In 2011, the Medicare Imaging Demonstration from the Centers for Medicare & Medicaid Services began testing whether exposing ordering clinicians to appropriateness guidelines for advanced imaging would reduce ordering inappropriate images. The evaluation examined trends in advanced diagnostic imaging utilization starting January 1, 2009-more than two years before the beginning of the demonstration-to November 30, 2013-two months after the close of the demonstration. Small changes in ordering patterns were noted, but decision support systems were unable to assign appropriateness ratings to many orders, thus limiting the potential effectiveness of decision support. Many opportunities to refine decision support systems have been identified.