This paper surveys the political career and personal life of Simon de Montfort. Derived largely from the author's biography of Montfort, it lays stress on his initial position as an outsider in English politics whose military abilities, diplomatic usefulness and personal charisma fostered his rise to power at the court of Henry III, but who subsequently fell out with the king and eventually became his fiercest opponent. It describes his position as lord of the honour of Leicester, from which town he expelled the Jews, and it goes on to assess the paradoxical and contrasting elements in Montfort's character, which combined deep piety and religious fervour with avarice and a self-seeking desire for his own and his family's advancement. It argues that personal differences, based largely on a strong sense of grievance and reflecting some of these character traits, rather than constitutional principles, lay behind his opposition to Henry III. It concludes by reviewing Montfort's role in the period of baronial reform and rebellion, 1258–65, and by describing his legacy, both to Henry III in his later years and to Edward I.