Monaco's Prince Albert II and his South African wife Princess Charlene have announced they are expecting their first child.
In a short statement, the couple - who married in July 2011 - expressed their "immense joy" at the news.
The statement said the birth was expected before the end of the year, but gave no further details.
The new baby will replace Princess Caroline - Prince Albert's older sister - as heir to the throne.
Prince Albert already has two children from previous relationships - 22-year-old daughter Jazmin and 10-year-old son Alexandre.
The tiny principality of Monaco, which covers about 2sq km (less than one square mile), is the world's smallest independent state after the Vatican City.
Prince Albert is the son of the late Prince Rainier III and actress Grace Kelly.
Princess Grace died after a car crash in 1982, and Prince Rainier died in 2005.
Prince Albert II has been Monaco's ruler since the death of his father.
He met Charlene Wittstock when he was presiding over a swimming race in 2000. She then went on represent South Africa in the 2000 Sydney Olympics.
They officially made their debut as a couple at the 2006 Winter Olympic Games in Turin, Italy.
Since their marriage, her official title is Her Serene Highness The Princess of Monaco.