Experience from Afghanistan and Iraq has demonstrated the vital nature of understanding human terrain, with conclusions relevant far beyond counterinsurgency operations in the Islamic world. Any situation where adversary actions are described as “irrational” demonstrates a fundamental failure in understanding the human dimension of the conflict. It follows that where states and their leaders act in a manner which in the U.S. is perceived as irrational, this too betrays a lack of human knowledge. This monograph offers principles for operating in the human domain which can be extended to consideration of other actors which are adversarial to the United States, and whose decisionmaking calculus sits in a different framework to our own — including such major states as Russia and China. This monograph argues that the human dimension has become more, not less, important in recent conflicts and that for all the rise in technology future conflicts will be as much defined by the participants’ understanding of culture, behavior, and language as by mastery of technology.