On any given night in the U.S., one in 10 youth aged 18-25 experience homelessness. The highest HIV risk groups disproportionately experience homelessness. Yet, YEH lack access to HIV prevention including highly effective pre- and post-exposure prophylaxis medications (i.e., PrEP, nPEP). Substance use and stress negatively impact HIV risk. Stress contributes to condomless sex, more sexual partners, and substance use with rates being twice that of housed youth. MY-RIDE, is a mobile health intervention that responds to Ecological Momentary Assessments to deliver personalized messaging and behavioral feedback. Pilot data found that MY-RIDE decreased drug use, sexual urges, and stress among YEH. This study will fully test MY- RIDE by conducting a randomized attention-controlled trial. MY-RIDE addresses HIV risk and its antecedents in real-time using messages developed using the Information-Motivation-Behavioral Skills model. Using Motivational Interviewing and Shared Decision Making approaches, nurse-led and on-demand sessions help YEH engage in HIV prevention despite their unstable housing circumstances. YEH receive personalized mobile messages that address their current HIV risk factors as well as substance use, mental health, and housing. Aim 1: Determine whether MY-RIDE decreases substance use compared to attention control youth (N=320; aged 18-25 years) at 3, 6, and 12 months. Aim 2: Determine whether MY-RIDE increases HIV prevention strategies (condom use, PrEP uptake, reduced STIs) when compared to attention control youth. Exploratory Aim 1: Evaluate MY-RIDE effects on willingness to take PrEP, stress, substance use urge, and use of mental health and substance use services when compared to attention control youth. This innovative study combines nurse-led counseling with JITAI technology to deliver personalized prevention supports at the time of heightened need. My-RIDE is a highly scalable intervention that increases reach to a disconnected population and facilitates delivery of an HIV prevention intervention that transcends barriers to access to care.