The overarching aim of the proposed dissertation research is to begin filling a major gap in research on youth who age out of the foster care system. Not only have these youth been removed from their families of origin, typically due to abuse or neglect, they often experienced multiple placement changes while in foster care. At age 18, when most American youth are still living at home or are in college, these vulnerable youth "age out" of the foster care system and are expected to assume adult roles with little or no support. The difficult life course experienced by these vulnerable youth places them at substantial risk of substance use/abuse, sexual risk-taking and mental health problems in adulthood. Although a few well-designed descriptive studies have documented these adverse outcomes, little is known about their etiology or the causal pathways that could be interrupted in order to produce more positive outcomes. There is theoretical reason to believe that placement stability while in foster care may be an important, but understudied factor. The specific aims of this study are to 1) describe substance use/abuse and sexual risk-taking in this vulnerable population, and 2) test theory-driven hypotheses about these outcomes including a) foster care placement instability is positively associated with early adult substance use and risky sexual behaviors; b) placement instability is positively associated with poor emotional and social well-being; and c) emotional and social well-being mediates the effect of placement instability on drug use and risky sexual behaviors. In addition, gender and ethnic differences in these relationships will be explored. The sample will include 300 young adults randomly sampled from the population of youth who aged out of the foster care system in the past three years in three Arizona districts. They will be interviewed via telephone to garner data to address the research questions. Additionally, data from the Arizona State Department of Economic Security will be abstracted to provide information on placement length and stability, and related variables. The public health significance of this research is to provide child welfare workers, health care providers, and policy makers with information that can be used to help reduce substance use/abuse, sexually transmitted diseases, unintended pregnancies, and mental health problems of youth who age out of the foster care system. [unreadable] [unreadable] [unreadable]