The growth in the use of advanced imaging for Medicare beneficiaries decelerated in 2006 and 2007, ending a decade of growth that had exceeded 6 percent annually. The slowdown raises three questions. Did the slowdown in growth of imaging under Medicare persist and extend to the non-Medicare insured? What factors caused the slowdown? Was the slowdown good or bad for patients? Using claims file data and interviews with health care professionals, we found that the growth of imaging use among both Medicare beneficiaries and the non-Medicare insured slowed to 1-3 percent per year through 2009. One by-product of this deceleration in imaging growth was a weaker market for radiologists, who until recently could demand top salaries. The expansion of prior authorization, increased cost sharing, and other policies appear to have contributed to the slowdown. A meaningful fraction of the reduction in use involved imaging studies previously identified as having unproven medical value. What has occurred in the imaging field suggests incentive-based cost control measures can be a useful complement to comparative effectiveness research when a procedure's ultimate clinical benefit is uncertain.