Emerging research examines biological processes not as primary causes of social outcomes but rather as mechanisms that themselves depend on social environments. In particular, environments that produce toxic stress help shape brain development and brain and body function throughout the lifespan. Early life stress, in particular, has serious consequences for life-long health and affects cognitive performance, emotional intelligence, and self-regulation. Because the brain is plastic, social as well as individual behavioral interventions can alter some of these developmental paths, modifying brain function and individual life trajectories—but with increasing difficulty as children become adolescents and adults. Now reflecting the new era of “epigenetics” and a life course perspective, this new view of stress, the brain, and social environments highlights the importance of the social, psychological, and biological sciences working together to elucidate underlying mechanisms both to expand knowledge and help promote a better society.