This chapter explores the history of military psychology and its influence on war. Beginning with World War I and continuing to today’s military operations, psychology has provided the military with better ways to select, train, develop, and lead soldiers in combat. Notable contributions of military psychology include aptitude testing, human factors engineering, clinical psychology, cyber technology, and positive psychology. Military psychologists may be civilians or uniformed members of all branches of service. They are employed in universities, government laboratories, hospitals, and nongovernment organizations including corporations and private consulting firms. The Society for Military Psychology is a founding division of the American Psychological Association. Given that the human element is the most important factor in warfare, military psychology is an essential science for winning the wars of today and tomorrow.