This paper, authored by James J. Heckman and Stefano Mosso, reviews and extends recent research on the economics of human development and social mobility. It highlights the importance of early life conditions in shaping multiple life skills and the critical and sensitive investment periods for different skill development. The authors present economic models that rationalize the evidence and unify the treatment effect and family influence literatures. They examine the empirical and policy importance of credit constraints in forming skills, finding limited support for the claim that untargeted income transfer policies significantly boost child outcomes. The paper emphasizes the essential role of mentoring, parenting, and attachment in successful families and interventions at all stages of childhood. It also discusses the dynamic complementarity of skills and investments, justifying policies that redistribute resources towards disadvantaged children in early years. The authors propose a dynamic state-space framework to operationalize the theory and unify the interpretation of intervention and family influence literature. The paper concludes with evidence on the effectiveness of interventions over the life cycle and interpretations using the proposed framework.This paper, authored by James J. Heckman and Stefano Mosso, reviews and extends recent research on the economics of human development and social mobility. It highlights the importance of early life conditions in shaping multiple life skills and the critical and sensitive investment periods for different skill development. The authors present economic models that rationalize the evidence and unify the treatment effect and family influence literatures. They examine the empirical and policy importance of credit constraints in forming skills, finding limited support for the claim that untargeted income transfer policies significantly boost child outcomes. The paper emphasizes the essential role of mentoring, parenting, and attachment in successful families and interventions at all stages of childhood. It also discusses the dynamic complementarity of skills and investments, justifying policies that redistribute resources towards disadvantaged children in early years. The authors propose a dynamic state-space framework to operationalize the theory and unify the interpretation of intervention and family influence literature. The paper concludes with evidence on the effectiveness of interventions over the life cycle and interpretations using the proposed framework.