Prevention and the concept of collective impact on population health is the focus of this issue led by Aradhana Bela Sood. Primary, secondary, and tertiary themes run throughout each article with evidence base explicitly stated. An Appendix presenting select programs for prevention concludes this issue. Topics include: Early childhood mental health: Neurobiological underpinnings of early brain development and Health promotion and prevention in non-psychiatric settings; Prevention in childhood; Mindfulness and alternative and complementary therapies; Prevention of violence; Bullying; Depression and suicide; HIV and AIDS; Substance use disorders; Obesity in children and youth; Delinquency and prevention; Public policy and system building. Some programs presented in the Appendix are Blueprint for violence prevention; Nurse-family partnership; Harlem Children's Project, and others.