This name was common among continental European royalty during the Middle Ages, and it was popularized in Britain by the German House of Hanover when they inherited the British throne in the 18th century. It was the name of characters in the novels Tom Jones (1749) by Henry Fielding and The Vicar of Wakefield (1766) by Oliver Goldsmith.
In the United States this name was only moderately common until the 1990s when it began rising in popularity, eventually becoming the most popular for girls from 2011 to 2013. A famous bearer is the Italian actress Sophia Loren (1934-).
This was the name of several dukes of Brittany, and Breton settlers introduced it to England after the Norman Conquest. Famous modern bearers include Alan Shepard (1923-1998), the first American in space and the fifth man to walk on the moon, and Alan Turing (1912-1954), a British mathematician and computer scientist.