His Serene Highness The Sovereign Prince of Monaco, ruled the Principality of Monaco for almost fifty-six years, making him one of the longest ruling monarchs of the 20th century. Though he was best known outside of Europe for having married American actress Grace Kelly, he was also responsible for reforms to Monaco's constitution and for expanding the principality's economy beyond its traditional gambling base. Gambling accounts for approximately three percent of the nation's annual revenue today; when Rainier ascended the throne in 1949, it accounted for more than 95 percent. Before his death, he was the world's second-longest reigning monarch with his 56 years of reign.