How can we use insights from cutting-edge science to improve the well-being and long-term life prospects of the most vulnerable children in our society? This is both a critical challenge and a powerful opportunity to affect the trajectories of millions of children in the United States and around the world. It is a question of particular importance to those who make or affect public policy. This paper shows how the science of child development can be leveraged to strengthen and improve the public child welfare system so that it can better support the children, families, and communities it serves. This paper has been developed in consultation with child welfare leaders and practitioners. Part I, The Science of Child Development, consolidates some of the key scientific concepts relevant to child welfare systems, including the science underlying child development, neglect, and the basic capabilities adults need in order to buffer the effects of adversity and promote healthy child development. Part II, Opportunities to Apply Developmental Science to Child Welfare, sets out five organizing principles for how these scientific findings can be applied to child welfare systems, providing examples to show how each principle can be used. The paper is intended for leaders in the public agencies responsible for child protection and related functions; in the private, non-profit agencies that provide many of the services in these systems; in the courts, which play a critical role in child welfare; in legislative committees that oversee child welfare and related services; and in the many other public systems, such as early childhood education, mental health, and juvenile justice, whose support is essential to success in child welfare. Additional references for the scientific material in this paper can be found in the three reports from the Center on the Developing Child listed on page 4 of this report. [The science in this report draws principally from the work of the National Scientific Council on the Developing Child. Additional funding was provided by the Hemera Foundation and the Palix Foundation.]