This is the first biography of one of the wealthiest and most influential bishops of medieval Europe, who for a period of over thirty years exercised a degree of power over the thirteenth-century Plantagenet court second only to that of the king. The career of Peter des Roches and the activities of his fellow aliens are fundamental to an understanding of the process by which England and France developed as two separate kingdoms. The book also sheds new light on such hotly-debated issues as the role of aliens in English politics, the reception of Magna Carta, the loss of Normandy, and the constitutional and administrative developments of the reign of Henry III.