This brief summarizes the working paper, "The Science of Early Childhood Development: Closing the Gap between What We Know and What We Do" (ED503049). In early childhood, research on the biology of stress shows how major adversity, such as extreme poverty, abuse, or neglect can weaken developing brain architecture and permanently set the body's stress response system on high alert. Science also shows that providing stable, responsive, nurturing relationships in the earliest years of life can prevent or even reverse the damaging effects of early life stress, with lifelong benefits for learning, behavior, and health. The brief also includes implications for policy and programs.