Scholars and analysts seeking to illuminate the extraordinary creativity and innovation evident in European medieval cultures and their afterlives have thus far neglected the important role of religious heresy. The papers collected here - reflecting the disciplines of history, literature, theology, philosophy, economics and law - examine the intellectual and social investments characteristic of both deliberate religious dissent such as the Cathars of Languedoc, the Balkan Bogomils, the Hussites of Bohemia and those who knowingly or unknowingly bent or broke the rules, creating their own ‘unofficial orthodoxies’. Attempts to understand, police and eradicate all these, through methods such as the Inquisition, required no less ingenuity. The ambivalent dynamic evident in the tensions between coercion and dissent is still recognisable and productive in the world today.