"The Bureau of Diplomatic Security manages the special protections and privileges afforded to foreign diplomats who reside in and visit the United States. Diplomatic Security ensures that these diplomats are treated fairly, that their embassies, consulates, and other diplomatic missions are well protected, and that they correctly receive and use their diplomatic and consular privileges. Our work not only helps the United States maintain good relations with foreign countries, it also helps ensure that foreign countries fairly treat our U.S. diplomats overseas. Of all countries, the United States hosts the highest number of persons who are entitled to some degree of diplomatic privilege and immunity, one of the oldest principles of foreign relations. As a matter of international law, our responsibility for serving diplomats and their embassies stems directly from the Vienna Conventions on Diplomatic Relations, signed in 1961, and on Consular Relations, signed in 1963. The foreign diplomatic community residing in the United States numbers more than 128,000 people--including diplomats, consular officers, and staff members of international organizations and their family members. Most of these persons are entitled to some degree of immunity; however, there are different categories of persons within a diplomatic mission, some of whom enjoy greater immunities and privileges than others. Only about 20,000 are given the highest level of diplomatic immunity. As with diplomats serving throughout the world, foreign diplomats living in the United States receive varying degrees of immunity from prosecution as well as tax, customs, and other benefits."
U.S. Government Printing Office, Federal Digital System: http://www.gpo.gov/fdsys/