Irregular warfare differs tremendously from the regular, conventional warfare for which Western armed forces traditionally have been optimized. This chapter explores this contrasting form of warfare and its underlying body of knowledge which characterizes irregular conflicts as violent struggles involving nonstate actors and states that seek to establish power, control, and legitimacy over relevant populations. Due to military asymmetry and the political nature of the struggle, the use of force mostly takes unconventional or unorthodox forms and is typically combined with other, non-kinetic, activities. As such, irregular warfare favors an indirect approach that does not focus on military defeat, but on winning the population(s) at stake and eroding the opponent's will. This forces conventionally focused armed forces to adapt to the specifics of an irregular conflict. In order to discuss the way western militaries have operationalized the irregular warfare mission, this analysis first describes terrorism and insurgency, the