Misogyny and 'toxic masculinity' are increasingly implicated in radicalisation. From the men's incel ('involuntary celibate') movement online, to jihadist groups like Islamic State, to radical right 'Free Speech' protests -radicalisation spans ideologies. Though an often-used term, the process of radicalisation is not well understood, and the role of gender and masculinities has often been ignored. This book uses primary research among two of Britain's key extremist movements: the banned Islamist group al-Muhajiroun, and those networked to it; and the anti-Islam radical right, including the English Defence League and Britain First, to reveal radicalisation as a masculinity project. Through interviews with leaders including Anjem Choudary, Jayda Fransen and Tommy Robinson, as well as their followers, Extreme Britain explores the emergence of extreme misogyny and masculinities. Pearson situates extreme identities in wider social norms, showing how masculinities are mobilised into action. The book cautions against oversimplifying extreme masculinity as 'toxic'. It demonstrates how both men and women 'do' extreme masculinities and the costs and benefits to them both of activism. Understanding the men and women involved in extreme movements will better equip us to counter them. This fascinating study offers invaluable insight into some of their lives and motivations.