The sovereign prince is the monarch and head of state of the Principality of Monaco. All reigning princes and princesses have taken the name of the House of Grimaldi. When Prince Rainier III died in 2005, he was Europe's longest reigning monarch. The Grimaldi family, which has ruled Monaco for eight centuries, is Europe's longest-ruling royal family.

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 or is selected by an established process from a family or cohort eligible to provide the nation's monarch. Alternatively, an individual may proclaim oneself monarch, which may be backed and legitimated through acclamation, right of conquest or a combination of means.