John of Gaunt, Duke of Lancaster (6 March 1340 – 3 February 1399), was an English prince, military leader and statesman. He was the fourth son (third surviving) of King Edward III, and the father of King Henry IV. Because of Gaunt's royal origin, advantageous marriages and some generous land grants, he was one of the richest men of his era and an influential figure during the reigns of both his father and his nephew, Richard II. As Duke of Lancaster, he is the founder of the royal House of Lancaster, whose members would ascend the throne after his death. His birthplace, Ghent in Flanders, then known in English as Gaunt, was the origin of his name.
John's early career was spent in France and Spain fighting in the Hundred Years' War. He made an abortive attempt to enforce a claim to the Crown of Castile that came through his second wife, Constance of Castile, and for a time styled himself as King of Castile. When Edward the Black Prince, Gaunt's elder brother and heir-apparent to the ageing Edward III, became incapacitated owing to poor health, Gaunt assumed control of many government functions and rose to become one of the most powerful political figures in England.