A principality (or sometimes princedom) can either be a monarchical feudatory or a sovereign state, ruled or reigned over by a regnant-monarch with the title of prince and/or princess, or by a monarch with another title considered to fall under the generic meaning of the term prince.

Rainier III (Rainier Louis Henri Maxence Bertrand Grimaldi; 31 May 1923 – 6 April 2005) was the Prince of Monaco from 9 May 1949 to his death in 2005. Rainier ruled the Principality of Monaco for almost 56 years, making him one of the longest-ruling monarchs in European history.

Rainier was born at the Prince's Palace of Monaco, the only son of Prince Pierre and Princess Charlotte of Monaco. He was crucially responsible for the transformation of Monaco's economy, shifting from its traditional casino gambling base to its current status as a tax haven and cultural destination. The Prince also coordinated the substantial reforms of Monaco's constitution, which limited the powers of sovereign rule.

Rainier married American film star Grace Kelly in 1956, which generated global media attention. They had three children: Caroline, Albert, and Stéphanie. He died in April 2005 from complications relating to a lung infection as a result of frequent smoking, and was succeeded by his son, Albert.