This chapter engages with the concept of identity-based violence, mapping its boundaries, articulating the phenomenon, and making the case for why this framing is useful for and in cities for prevention. It offers shared roots and risk factors of identity-based violence before considering the people, systems, transactions, and behaviors that implement or enable identity-based violence. It considers how identity-based violence evolves, intensifies, gathers momentum, and becomes systematic and or widespread, and therefore mass. It then turns to how such violence can be slowed, halted, or prevented within the city. The chapter ends by looking to the future of preventing identity-based violence in cities, identifying common threats and opportunities. Throughout, this chapter investigates the implications of identity-based violence in cities, drawing on Protection Approaches' work in London and the UK over the past decade and their mission to transform understanding and prevention of identity-based violence.