In a monarchy, a regent () is a person appointed to govern a state pro tempore because the monarch is a minor, absent, incapacitated or unable to discharge the powers and duties of the monarchy, or the throne is vacant and the new monarch has not yet been determined. One variation is in the Monarchy of Liechtenstein, where a competent monarch may choose to assign regency to their of-age heir, handing over the majority of their responsibilities to prepare the heir for future succession. The rule of a regent or regents is called a regency. A regent or regency council may be formed ad hoc or in accordance with a constitutional rule. Regent is sometimes a formal title granted to a monarch's most trusted advisor or personal assistant. If the regent is holding their position due to their position in the line of succession, the compound term prince regent is often used; if the regent of a minor is their mother, she would be referred to as queen regent.
If the formally appointed regent is unavailable or cannot serve on a temporary basis, a regent ad interim may be appointed to fill the gap.
In a monarchy, a regent usually governs due to one of these reasons, but may also be elected to rule during the interregnum when the royal line has died out. This was the case in the Kingdom of Finland and the Kingdom of Hungary, where the royal line was considered extinct in the aftermath of World War I. In Iceland, the regent represented the King of Denmark as sovereign of Iceland until the country became a republic in 1944. In the Polish–Lithuanian Commonwealth (1569–1795), kings were elective, which often led to a fairly long interregnum. In the interim, it was the Roman Catholic primate (the archbishop of Gniezno) who served as the regent, termed the interrex (Latin: ruler 'between kings' as in ancient Rome). In the small republic of San Marino, the two Captains Regent, or Capitani Reggenti, are elected semi-annually (they serve a six-month term) as joint heads of state.