Trauma and Socioemotional Development in Foster Youth:Lessons Learned from LONGSCAN: Understanding the risks that foster children and youth face at various times and how these risks impact their socioemotional development is critical to informing policy and practice. After briefly describing the five-site, now 27-year prospective Longitudinal Studies of Child Abuse and Neglect (LONGSCAN) Consortium, and the unique features of the San Diego LONGSCAN Foster Care Study site, in this chapter we present what we have learned about: 1) characterizing maltreatment; 2) the myriad other adversities experienced by children and youth who are maltreated and/or involved with the Child Welfare System; 3) how maltreatment and these adversities are related to Child Welfare System practices; and ultimately 4) how maltreatment, other adversities, and Child Welfare System practices are related to developmental outcomes. Based on both quantitative and qualitative findings from this work, we attempt to point out their implications for Child Welfare System research, practice, and policy as we move forward.