What factors make it possible for mafia groups to move successfully to new geographic regions? In his fascinating new book, Mafias on the move: how organized crime conquers new territories, Federico Varese develops a theory of mafia migration and uses it to explain numerous cases around the world. This lively book expands our knowledge of the nature of mafia groups, the potential impacts of a more globalized world, and the importance of informal, non-state based institutions in both developed and developing economies. Beginning with landmark work by Diego Gambetta (1993) on the Sicilian Mafia, a growing literature by economists, criminologists, and sociologists on mafias has argued that their defining characteristic is to provide protection when the state cannot or will not do so. Mafias supply extra-legal governance institutions. This includes private protection to both legal and illegal businesses, enforcement of agreements among people who cannot rely on state-based institutions, such as courts of law, and the organization of illegal markets. Most of this literature is empirical, describing the origin of these groups, their organization, and assessing the effects of their operations. Varese begins the book from an analytical perspective by outlining a theory to explain the phenomena of mafia transplantation: when a mafia group can “operate an outpost over a sustained period outside its region of origin and routine operation.” Varese identifies both supply and demand conditions that facilitate mafia transplantation. On the supply side, mafia members must migrate to new regions, which often results as members flee law enforcement or when excommunicated members seek safer surroundings. Laborers willing to engage in violence must exist in the region and desire to work for mafias. The existence or absence of other mafia groups in the region determines whether market conditions are suitable for an additional mafia group. Mafias will be less likely to transplant if the region has an established and effective mafia group already in operation.