An interregnum is a period between kings, or between popes of the Roman Catholic Church. In particular:

• The 1254-1273 period in the Holy Roman Empire between the end of Hohenstaufen rule and the beginning of Habsburg rule.
• The 1332-1340 period in Denmark when the country was mortgaged to a few German counts.
• The 1383-1385 Crisis in Portugal
• The 1402-1413 Ottoman Interregnum
• The 1481–1483 in Norway
• The 1649-1660 republican period in Britain, comprising the Commonwealth and the Protectorate of Oliver Cromwell after the regicide of Charles I and before the restoration of Charles II

In some monarchies, such as the United Kingdom, an interregnum is usually avoided due to a rule described as "the king is dead, long live the king", i.e. the heir to the throne becomes a new monarch immediately on his predecessor's death or abdication. This is not so in other monarchies where the new monarch's reign begins only with coronation or some other formal or traditional event. In the Polish-Lithuanian Commonwealth for instance, kings were elected, which often led to relatively long interregna. During that time it was the Polish primate who served as an interrex (ruler between kings).

An interregnum occurs also upon the death of the Roman Catholic Pope, though this is generally known as a sede vacante (vacant seat). The interregnum ends immediately upon election of the new Pope by the College of Cardinals.