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 otherwise incapable of ruling. 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 (e.g. 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 (e.g. Sovereign Prince of Monaco), emperor or empress (e.g. Emperor of Japan, Emperor of India), Shah of Iran, or duke or grand duke (e.g. 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 monarchs have been male, female monarchs have also ruled in history; the term queen regnant refers to a ruling monarch, as distinct from a queen consort, 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 become 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 illustrated in the classic phrase "The [old] King is dead. Long live the [new] King!").

A form of government may in actual fact be hereditary without being considered monarchy, such as family dictatorship or political families present in some nominally democratic countries.

The word monarch (Latin: monarcha) comes from the Greek μόναρχος (from μόνος, one, singular and ἀρχός, leader, guide, chief) which referred to a single, at least nominally absolute ruler. With time, the word has been succeeded in this meaning by others, such as autocrat or dictator. In modern usage the word monarch is generally used when referring to a traditional system of hereditary rulership, with elective monarchies often considered as exceptions.

A particular case is the French co-prince of Andorra, a position held by the elected President of France. Nonetheless, he is still generally considered a monarch because of the traditional use of a monarchical title (even though Andorra is, strictly speaking, a diarchy.) Similarly, the Yang di-Pertuan Agong of Malaysia is considered a monarch despite only holding the office for five years at a time. On the other hand, several life-time dictators around the world have not been formally classified as monarchs, even if succeeded by their children, but that may be more to do with international political sensitivities than with semantics.

Hereditary succession within one family has been most common. The usual hereditary succession is based on some cognatic principles and on seniority, though sometimes merit has played a part. Thus, the most common hereditary system in feudal Europe was based on cognatic primogeniture where a lord was succeeded by his eldest son or, if he had no son, by either daughters or sons of daughters. The system of tanistry was semi-elective and gave weight also to merits and capability.

The Quasi-Salic succession provided firstly for male members of the family to succeed, and secondarily males descended from female lines. In most feudal fiefs, females (such as daughters and sisters) were allowed to succeed, should the male line fail, but usually the husband of the heiress became the real lord and most often also received the title, jure uxoris. Great Britain and Spain today continue this model of succession law, in the form of cognatic primogeniture. In more complex medieval cases, the sometimes conflicting principles of proximity and primogeniture battled, and outcomes were often idiosyncratic.