Orders of Knighthood, in modern usage, honorary societies, membership in which is generally conferred by a nation or a head of state on nobility or persons who have performed some unusually meritorious service. Three categories of orders may be distinguished: royal orders, generally limited to noblemen and women of royal blood or of the highest grade; noble or family orders, open to nobility generally; and orders of merit, bestowed on persons of all classes as a reward for distinguished service. Persons admitted to membership in an order generally receive a badge or medal and a formal title such as Knight Commander or Knight of the Grand Cross. The orders are believed to have originated during the age of feudalism, when many of the nobility, impoverished by their heavy expenditures during the Crusades or virtually disinherited through the operation of the law of primogeniture (inheritance right of the firstborn son), became professional soldiers in the service of various kings or other nobles. These knights formed organizations known as orders, which had special uniforms and insignia and were given distinctive names. Of the profusion of orders created in the Middle Ages, several remain in existence. These include the Order of the Garter (Great Britain), the Order of the Elephant (Denmark), and the Order of the Golden Fleece (Spain, formerly shared with Austria). Modern orders use the symbols and nomenclature of the feudal orders, but serve a purely honorary function.

In modern times, orders of knighthood have been created and conferred most often by monarchies, a few empires, and the Holy See of the Roman Catholic Church. Orders were abolished, in some instances by law, in such countries as Austria, Germany, and the former Union of Soviet Socialist Republics (USSR).

Although certain religious orders such as the Knights of Saint John of Jerusalem, fraternal or service orders such as the Knights of Columbus, and various labor organizations and secret societies employ the word knight in their names, they are in no way related to the orders of knighthood.