This book explores the causes, progression and consequences of the extraordinary spread of anti-Jewish violence and mass conversion across five separate Spanish polities in 1391, from Seville to the Pyrenees, overwhelming Valencia, Barcelona and numerous other locations. Using comparative analysis with previous outbreaks in Spain and elsewhere, it demonstrates the uniqueness of these events in terms of the speed and extent of transmission of attacks, and their lasting consequences. It argues that models of social contagion best explain this pandemic violence, in which latent hostilities, fears and uncertainties in the post-Black Death world, national and local tensions, were almost spontaneously triggered into often annihilatory riots by rapid communication and movement of people, spreading ideas, news, gossip and rumour through a variety of social networks. It seeks to demonstrate the modes by which polemic and tropes were translated into action, by local preachers, poetry, troubadours and the visual arts.