Constantine I was King of the Hellenes and Prince of Denmark and Norway.
He was born on 2 August 1868 in Athens, Greece, son of King George I of the Hellenes and Grand Duchess Olga Constantinovna of Russia. He became King of the Hellenes upon the death of his father on 18 March 1913, and reigned for the rest of his life. He died on 11 January 1923 in Athens.
He married on 27 October 1889 to Princess Sophia of Prussia (1870-1932), daughter of Frederick III, German Emperor and King of Prussia, and Princess Victoria of Great Britain and Ireland, having the following issue:
George (1890-1947), who succeeded his father as King George II of the Hellenes.
Helen (1896-1982), who was married 1921-1928 to King Charles II of the Romanians.
Paul (1901-1964), who succeeded his brother as King Paul I of the Hellenes