After doctors finish their education and postgraduate specialty training, many of them take an examination given by a board of experts that certifies their knowledge in the field. Those who pass the examination are then known as board-certified specialists. Examinations can also be taken for certification in subspecialties and in areas of added qualification. In the past, once a doctor was certified, many types of board certification never had to be renewed. More than a decade ago, however, the certifying boards decided that periodic reexamination should be done to ensure that doctors were keeping up with current medical knowledge. Beginning in 1990, the American Board of Internal Medicine began issuing certificates that expired after 10 years. Maintenance of Certification (MOC) requires continuing study and passing a proctored examination every 10 years thereafter. Not all doctors who were initially board certified have taken part in the MOC process.