John of Gaunt, Duke of Lancaster (6 March 1340 – 3 February 1399) was an English royal prince, military leader, and statesman. He was the third surviving son of King Edward III of England, and the father of King Henry IV. Due to Gaunt's royal origin, advantageous marriages, and some generous land grants, he was one of the richest men of his era, and was 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. When he became unpopular later in life, a scurrilous rumour circulated, along with lampoons, claiming that he was actually the son of a Ghent butcher. This rumour, which infuriated him, may have been inspired by the fact that Edward III had not been present at his birth.
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 due to poor health, Gaunt assumed control of many government functions, and rose to become one of the most powerful political figures in England. He was faced with military difficulties abroad and political divisions at home, and disagreements as to how to deal with these crises led to tensions among Gaunt, the English Parliament, and the ruling class, making him an extremely unpopular figure for a time.
John exercised great influence over the English throne during the minority of King Richard II (Edward the Black Prince's son) and the ensuing periods of political strife. He mediated between the king and a group of rebellious nobles, which included Gaunt's own son and heir-apparent, Henry Bolingbroke.