A monarchy is a form of government in which a person, the monarch, is head of state for life or until abdication. The political legitimacy and authority of the monarch may vary from restricted and largely symbolic (constitutional monarchy), to fully autocratic (absolute monarchy), and can span across executive, legislative, and judicial domains.

The succession of monarchs in many cases has been hereditical, often building dynastic periods. However, elective and self-proclaimed monarchies have also been established throughout history. Aristocrats, though not inherent to monarchies, often serve as the pool of persons to draw the monarch from and fill the constituting institutions (e.g. diet and court), giving many monarchies oligarchic elements.

Monarchs can carry various titles such as emperor, empress, king, and queen. Monarchies can form federations, personal unions and realms with vassals through personal association with the monarch, which is a common reason for monarchs carrying several titles.

Monarchies were the most common form of government until the 20th century, by which time republics had replaced many monarchies. Today forty-three sovereign nations in the world have a monarch, including fifteen Commonwealth realms that share King Charles III as their head of state. Other than that, there is a range of sub-national monarchical entities. Most of the modern monarchies tend to be constitutional monarchies, retaining under a constitution unique legal and ceremonial roles for the monarch, exercising limited or no political power, similar to heads of state in a parliamentary republic.