Rio de Janeiro, along with other Latin American cities, are in the top of global rankings of cities facing high rates of chronic urban violence, of which men are the main perpetrators as the vast majority of homicide victims. Dominant, hyper-masculine or masculinist norms that uphold violence represent a shared characteristic of state-sanctioned and criminal groups in Brazil – including drug trafficking gangs,[i] militia (mostly comprised of off-duty police) and police forces. Understanding that these hyper-masculine norms are constructed during the socialization of boys, and continue to be reinforced as men are exposed to groups that use armed violence can offer insightful strategies to reducing urban violence. This article presents findings from two studies carried out Promundo starting in 1999. Promundo is a Brazilian-based NGO, which now works in more than 20 countries, that carries out applied research, program development and advocacy related to gender equality and violence prevention.