Conflict in the 21st Century differs markedly from the way the United States thought about and prepared for war and defense during most of the 20th Century. Armed Groups remain key actors, having strategic impact at the local, state, regional, and even global level. As these implications come into sharper focus, change is taking place in military doctrine and operations. To prepare tomorrow's warriors and their intelligence counterparts for these challenges, the professional military education program (PME) of the US Department of Defense also needs to adapt to the new strategic context. To assist in the development of this education, in 2005 the National Strategy Information Center initiated a project to develop curricula about the armed group phenomenon, and to prepare PME faculty to teach the subject in the nation's professional military schools, colleges, and universities. NSIC has been at the forefront of innovating and institutionalizing education on major dimensions of security studies with military and civilian partners for more than 40 years. The new curriculum was developed in consultation with the leadership of America's top military schools, and was refined at two week-long seminars for military and civilian faculty.The authors, all experienced faculty, present the dimensions of the 21st century security environment, and describe the evolution of the curricula development. This groundbreaking syllabus on Armed Groups and Irregular Warfare is in five parts, each with defined learning objectives, substantive subject matter, and annotated references of the key literature.The 21st century global security environment and the key role of armed groups Types of armed groups; insurgents, militias, terrorists, and organized crime Profiling armed groups and movements: A new approach to the order of battle Strategic, regional, and indirect threats from armed groups and opportunities Meeting the Challenge: the whole of government approach