This systematic review synthesizes effectiveness evidence on interventions to prevent and address youth homelessness. It was conducted primarily for a United States policy and practice audience but involved an international synthesis of evidence. The authors conducted an international search that included eleven major academic electronic databases, 13 additional relevant institutional web-based publication databases, and a professional outreach for published and unpublished studies of the effectiveness of programs and practices to prevent or address youth homelessness, in Organization for Economic Co-operation and Development (OECD) countries. We searched databases for studies published or completed between January 1, 2008 and March 19, 2019, and we also reviewed earlier studies synthesized in a similar past systematic review that ended its search in 2008. The review included experimental and quasi-experimental evaluation studies of youth ages 13–25. We included studies that only used pre-post comparisons and denote lower rigor designs clearly in the synthesis.