“Prevention in psychiatry is possible. A scientific base of evidence shows that we can prevent many mental, emotional, and behavioral disorders before they begin.” The introduction to this month's lead article reminds professionals that they can draw on their knowledge and experience not only to improve outcomes for their patients with mental illnesses but also to reduce people's risk of developing mental illnesses. William R. Beardslee, M.D., and coauthors highlight and expand key points from a landmark 2009 Institute of Medicine (IOM) report titled Preventing Mental, Emotional, and Behavioral Disorders Among Young People: Progress and Possibilities. Because half of all lifetime cases of mental, emotional, and behavioral disorders start by age 14—and symptoms typically occur two to four years before diagnosis—prevention focuses primarily on young people. The authors discuss evidence for prevention from a developmental perspective, including biological and psychosocial risk factors, developmental plasticity, and preventive interventions, such as parenting programs and interventions for groups at high risk of depression. Dr. Beardslee and colleagues highlight the cost savings that can be achieved with prevention, which are multiplied over the lifetimes of affected youths. The authors summarize two sets of IOM recommendations for incorporating prevention principles into practice—one for individual clinicians and the other for mental health systems. They call on clinicians to envision and work toward a future in which the health care system is organized on these principles (page 247).