Non–state violent actors (NSVAs) challenge the state apparatus, with some observers arguing that future wars will be fought against sub–national groups. Gangs are one type of NSVA that has, for the most part, been excluded from this discourse. On the one hand, gang research by criminologists primarily focuses on crime and deterrence. From this perspective, the gang threat is localized with little, if any, attention given to their national security implications. On the other hand, gang research in international studies is either limited to weak and failing states or altogether ignored in favor of insurgent and terrorist groups. From this perspective, enemy combatants are defined in military terms with a focus on NSVAs motivated by state subversion. The purpose of this book is to examine the security implications of gang networks in the United States and the extent to which they challenge law and order, weaken institutions, and undermine the structural integrity of the state. A significant aspect of the evolving security threat landscape is the emergence of gang variations, their ability to operate across unbounded geographic spaces, and their network connections, which act as a threat multiplier.