The early modern period in Europe was dominated by expansionist empires often composed of loose collaborations of ethnic or national groupings, with only a limited level of state unity and centralized authority. Instead of being divided by clearly demarcated borders, semiautonomous border zones existed in geographic areas that acted as natural boundaries (rugged mountains, stretches of uninhabitable wilderness, and coastlines). In many instances, as illustrated by the three border zones profiled in this article, state authorities recognized the limits of their resources in controlling border lands and entered into a mutually beneficial arrangement with populations or communities established in the border zones. These communities agreed to act as border guards, defending the border territory from military invasion and protecting the frontier zone against criminal raids. In return, the state recognized the independent status of these communities, freeing them from being serfs to those wielding power in the state's interior. This made conditions in these frontier zones conducive to a high degree of criminality. The evidence presented in this article documents that such crime was sustained and structured, taking it to a level beyond petty theft or occasional banditry. Criminals who operated in the border regions worked as small, loosely organized groups engaged in sustained smuggling, banditry, piracy, armed robbery, and extortion under the threat of violence. These criminal groups formed an "underworld" that had customs, institutions, identities, and a culture that differed sharply from that of mainstream society in these border communities. 66 notes