Mental illness and substance use disorders (SUDs) are strong risk factors for homelessness, regardless of Veteran status. This chapter describes the interplay between mental illness, SUDs, the co-occurrence of these disorders (CoD), and homelessness among Veterans; the authors consider military-specific experiences (e.g., combat) and factors associated with homelessness for both Veteran and non-Veteran adults (e.g., childhood adversity). They present epidemiology data on mental illness and SUDs among homeless Veterans, considering specific diagnoses (e.g., post-traumatic stress disorder) and experiences (e.g., military sexual trauma) that are highly prevalent within or unique to persons who served in the United States Armed Forces. In addition, the authors present qualitative and quantitative data on pathways to homelessness for Veterans with mental illness, SUDs, and/or CoD, considering distinct pathways seen in special populations (e.g., women Veterans, who have high rates of trauma, and the newest cohort of Veterans who served in Iraq or Afghanistan). Last, the authors discuss supported housing outcomes for homeless Veterans with psychiatric disorders and SUDs. They discuss the utility of these programs for homeless Veterans with mental health problems and explore the influence of psychiatric disorders and SUDs among Veterans with poor supported housing outcomes.