The approach that various countries have taken to regulate the marketing of medical devices differs, greatly as does their use of standards for medical devices. The United States has the most complex system because of its highly detailed legislation and the numerous amendments to that legislation. Whereas the European Community has adopted a method-based certification of conformance to standards, the U.S. rules do not require conformance to standards. The complexity of the U.S. system is because of its evolution from a law that presumed medical devices being marketed before enactment of the law are reasonably safe and effective unless proven otherwise. Most new devices enter the U.S. market based on “substantial equivalence” to devices that are presently being marketed. Except for devices that have been exempted, the specifications of all devices entering the U.S. market are examined by the U.S. Food and Drug Administration.