The science of child development shows that the foundation for sound mental health is built early in life, as early experiences—which include children’s relationships with parents, caregivers, relatives, teachers, and peers—shape the architecture of the developing brain. Disruptions in this developmental process can impair a child’s capacities for learning and relating to others, with lifelong implications. This summary explains why many costly problems for society, ranging from the failure to complete high school to incarceration, could be dramatically reduced if attention were paid to improving children’s relationships and experiences early in life.