A monarch is the person who heads a monarchy, a form of government in which the country or entity usually ruled or controlled by an individual who usually rules for life or until abdication. Monarchs may be autocrats (absolute monarchy) or may be ceremonial heads of state who exercise little or no power or only reserve power, with actual authority vested in a parliament or other body (constitutional monarchy). Most states only have a single monarch at any given time, although a regent may rule when the monarch is a minor, not present or debilitated. Two monarchs have ruled simultaneously in some countries, as in the ancient Greek city-state of Sparta or the joint sovereignty of spouses or relatives (eg. William and Mary of Kingdom of England and Scotland, Peter and Ivan of Russia, Charles and Joanna of Castile, etc).

Monarchs have various titles — king or queen, prince or princess (eg. Sovereign Prince of Monaco), emperor or empress (eg. Emperor of Japan, Emperor of India), or even duke or grand duke (eg. Grand Duke of Luxembourg). Many monarchs are distinguished by titles and styles. They often take part in certain ceremonies, such as a coronation.

Monarchy are associated with political or sociocultural in nature hereditary rule; most monarchs, both historically and in the modern day, have been born and brought up within a royal family (over a period of time called a dynasty) and trained for future duties. Different systems of succession have been used, such as proximity of blood, primogeniture, and agnatic seniority (Salic law). While traditionally most monarch have been male, female monarchs have also ruled in history; the term queen regnant refers to a ruling monarch, while a queen consort refers to the wife of a reigning king.

Some monarchies are non-hereditary. In an elective monarchy, the monarch is elected but otherwise serves as any other monarch. Historical examples of elective monarchy include the Holy Roman Emperors (chosen by prince-electors but often coming from the same dynasty) and the free election of kings of the Polish–Lithuanian Commonwealth. Modern examples include the Yang di-Pertuan Agong of Malaysia and the pope of the Roman Catholic Church, who serves as Sovereign of the Vatican City State and is elected to a life term by the College of Cardinals.

Monarchies have existed throughout the world, although in recent centuries many states have abolished the monarchy and becomes republics. Advocacy of republics is called republicanism, while advocacy of monarchies is called monarchism. The principal advantage of hereditary monarchy is the immediate continuity of leadership, with a usually short interregnum (as seen in the classic phrase "The King is dead. Long live the King!").

Form of governments may be hereditary without being considered monarchies, such as that of family dictatorships or political families are present in many democracies.