Early childhood is a time of great promise and rapid change, when the architecture of the developing brain is most open to the influence of relationships and experiences. Yet, at the same time, significant disadvantages in the life circumstances of young children can undermine their development, limit their future economic and social mobility, and thus threaten the vitality, productivity, and sustainability of an entire country. A remarkable expansion of new knowledge about brain development in the early years of life, linked to advances in the behavioral and social sciences, is now giving us deeper insights into how early experiences are built into our bodies, with lasting impacts on learning, behavior, and both physical and mental health. Tese insights can be used to fuel new ideas that capitalize on the promise of the early years and lead to breakthrough solutions to some of the most complex challenges facing parents, communities, and nations.