This is a book by and about military research psychologists-behavioral scientists who apply the principles and methods of psychology toward the goal of understanding and improving human health and performance in our defense forces. This group of specialists, along with their counterparts in the Army's sister Services and the many civilian research psychologists throughout the Department of Defense (DOD), forms a critical and essential core asset for our current and future national security. Since World War II, we have relied increasingly on research psychologists within DOD to design better and more efficient selection and classification tools and strategies, more effective training programs for military personnel, and improved leader development programs. In addition, these specialists have applied rigorous research methods to understanding the limits and capacities of human mental and physical performance in a range of demanding operational environments. Their work has provided much of the knowledge that has allowed senior national security leaders to design systems and policies that appreciate and capitalize on our human talent, rather than ignore or frustrate it. Some of this research relies upon the use of animal models, an approach that has proven essential for understanding and developing treatments for chemical defense, stress, and traumatic brain injuries. More recently, military research psychologists have applied their expertise to understanding the causes and cures of stress-related injuries in troops, such as post-traumatic stress disorder, traumatic brain injury, drug and alcohol problems, and even suicide. This work being done by research psychologists not only helps with operational effectiveness, but it also helps the individuals who are put under great stress during military operations to cope with that pressure; it can save lives and enhance well-being. Military research psychologists are responsible for significant advances in understanding human resilience and hardiness under stress, knowledge that is being applied in the design of more effective training and prevention programs. As the military operations that we are engaged in as a nation and as allies grow in complexity, understanding the human dimension is more essential than ever. No matter how good our technologies may be, technology alone will not win the day in the challenges before us. It is the quality, strength, intelligence, and morality of our human systems that ultimately will allow us to realize the shared goal of a world in which security and prosperity are the normal state for all inhabitants. The contributions of research psychologists such as those in this volume are critical for moving us toward that goal.