Homelessness among US veterans has been a focus of research for over 3 decades. Following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, this is the first systematic review to summarize research on risk factors for homelessness among US veterans and to evaluate the evidence for these risk factors. Thirty-one studies published from 1987 to 2014 were divided into 3 categories: more rigorous studies, less rigorous studies, and studies comparing homeless veterans with homeless nonveterans. The strongest and most consistent risk factors were substance use disorders and mental illness, followed by low income and other income-related factors. There was some evidence that social isolation, adverse childhood experiences, and past incarceration were also important risk factors. Veterans, especially those who served since the advent of the all-volunteer force, were at greater risk for homelessness than other adults. Homeless veterans were generally older, better educated, and more likely to be male, married/have been married, and to have health insurance coverage than other homeless adults. More studies simultaneously addressing premilitary, military, and postmilitary risk factors for veteran homelessness are needed. This review identifies substance use disorders, mental illness, and low income as targets for policies and programs in efforts to end homelessness among veterans.