<i>Objectives: </i>The Vancouver At Home (VAH) Study is part of a multi-site Canadian program of research that seeks policy relevant evidence surrounding service interventions for adults who are homeless and mentally ill. This paper provides an overview of the ocal VAH study design, including demographic and mental health characteristics of the baseline sample. <i>Participants: </i>Eligible participants included those aged 19 years or older, with a serious mental illness and who lacked a regular, fixed shelter or whose primary residence was a single room occupancy, rooming house, or hotel/motel. Participants were randomly assigned to a housing intervention or treatment as usual, based upon level of need. <i>Setting: </i>Individuals were recruited for participation through referrals from a wide variety of agencies throughout Vancouver. <i>Intervention: </i>Participants with high needs were randomized to Housing First with Assertive Community Treatment, Congregate Housing with on-site supports or treatment as usual. Participants with moderate needs were randomized to Housing First with Intensive Case Management or treatment as usual. <i>Outcomes:</i> A majority of individuals in the high needs group presented with psychotic disorder and substance dependence problems while a substantial minority met criteria for major depression. Among the moderate needs group, just over half of the sample met criteria for major depression and substance dependence with one-third of the group meeting criteria for post-traumatic stress disorder. <i>Conclusion: </i>The characteristics of the sample provide insight into the breadth and differential patterns of mental health problems facing homeless individuals and speak to the need for interventions that meet the broad service needs of this vulnerable population.