PROJECT SUMMARY/ABSTRACT In the United States, it is estimated that 1 in 30 adolescents and 1 in 10 young (“emerging”) adults experienced homelessness in the past year. Collectively, youth experiencing homelessness (YEH) are a highly vulnerable population who are at high risk of experiencing multiple negative health and psychosocial outcomes. In particular, the prevalence of substance use and substance use disorders among YEH is exceedingly high. Despite this, only a minority of YEH with substance use disorders have received some form of substance use treatment. This may be due to multiple barriers to receiving treatment, including lack of perceived need, transportation and logistical difficulties, and lack of insurance or ability to pay. For YEH who do receive substance use treatment, research indicates that a variety of substance use interventions may be effective, although research to date has largely been conducted in drop-in center and shelter settings. There is little research available that uses nationwide treatment data sets or qualitative methods to describe the unique characteristics or treatment outcomes of YEH. The proposed dissertation research uses an ecological- developmental approach to investigate these issues among emerging adults (18-25) experiencing homelessness. This age group makes up 89% of YEH in the U.S. Developmental theory such as the Risk Amplification Model posits that YEH have experienced multiple adversities before and after becoming homeless, and that these adversities are associated with risky health behaviors and harmful developmental outcomes. Furthermore, the Theory of Emerging Adulthood posits that the young adult years (ages 18-25) are a period characterized by greater independence, instability, and opportunity to prepare for adult roles. Emerging adult factors such as greater independence and peer influences have been used to explain heightened substance use during this stage. However, emerging adulthood theory has rarely been used to inform research with YEH. The proposed research has three aims. Aims 1 and 2 will utilize quantitative analysis of a nationwide treatment data set (the Global Appraisal of Individual Needs). Aim 1 will describe the prevalence of recent homelessness and identify the unique characteristics of emerging adults experiencing homelessness in substance use treatment. Aim 2 will examine longitudinal associations between housing stability, ecological-developmental factors, and substance use outcomes during and after substance use treatment. Aim 3 will utilize in-depth qualitative interviews with emerging adult YEH who meet substance use disorder criteria (n = 10) and homeless youth service providers (n = 10) in Los Angeles, California. Together, this research will expand knowledge regarding the needs, characteristics and substance use outcomes of emerging adult YEH in treatment, allowing for the development of tailored treatment and housing interventions. In addition, the PI will receive...