A monarch is a head of state for life or until abdication, and therefore the head of state of a monarchy. A monarch may exercise the highest authority and power in the state, or others may wield that power on behalf of the monarch. Usually a monarch either personally inherits the lawful right to exercise the state's sovereign rights (often referred to as the throne or the crown) or is selected by an established process from a family or cohort eligible to provide the nation's monarch. Alternatively, an individual may proclaim themself monarch, which may be backed and legitimated through acclamation, right of conquest or a combination of means.

If a young child is crowned the monarch, then a regent is often appointed to govern until the monarch reaches the requisite adult age to rule. Monarchs' actual powers vary from one monarchy to another and in different eras; on one extreme, they may be autocrats (absolute monarchy) wielding genuine sovereignty; on the other they may be ceremonial heads of state who exercise little or no direct power or only reserve powers, with actual authority vested in a parliament or other body (constitutional monarchy).

A monarch can reign in multiple monarchies simultaneously. For example, the monarchy of Canada and the monarchy of the United Kingdom (as well as 14 other Commonwealth realms) are separate states, but they share the same monarch through personal union.