This research brief was developed as part of a project to summarize research available on youth who experience prolonged homelessness. While there are well-established connections between adult chronic homelessness and serious mental illness, the relationship between mental illness and youth is less understood. However, research suggests that mental health disorders such as psychosis that first become symptomatic during adolescence and early adulthood can increase risk for homelessness. Mental health disorders such as mood disorders, depression, post-traumatic stress disorder, and suicidal ideation are far more prevalent in youth experiencing homelessness than the those in the general population. In addition, the more time youth spend on the streets, the more likely they are to experience mental health disorders. Early interventions for first-episode psychosis and supportive housing programs may help prevent and address prolonged homelessness among young people with mental health issues. Further research is needed to understand the relationship between serious mental illness and homelessness and what types of interventions are most effective for supporting youth and young adults with serious mental illness to live healthy and productive lives.