In this chapter, students will learn about the theory and practice of counterinsurgency. This is a military strategy that has gone in and out of fashion over time. Early in this century counterinsurgency achieved new currency with the launching of military operations in Afghanistan and Iraq that were met by stiff local resistance. The difficulties of fighting enemies who hid among the people and avoided classic military confrontations led to a new generation of writings on counterinsurgency operations, which were swiftly put into practice in the field. However, in the US, counterinsurgency’s star is currently waning as politicians seek to avoid long-term military interventions and the US military prioritizes planning for major wars against high-technology enemies. Nevertheless, the parallel skill sets between peacekeeping and counterinsurgency will ensure that the practice will retain relevance for many militaries. In significant contrast to other areas of security studies, counterinsurgency is an issue area where there are many scholar-practitioners, which gives their writings particular immediacy and applicability.