Housing First was developed to meet the needs of homeless individuals with serious mental illness and substance use challenges. The overlap with the top one percent of individuals using a costly site of health care but whose needs are better met in the community is significant. As many as 70 percent of these individuals have a mental illness, 50 percent have a SUD, as many as one-third are dealing with opioid dependence and close to half are homeless. Housing First approaches that target this high-need, high-cost population are among the most cost-effective.^{35,36,37,38,39}