Abstract   Adults with both a mental illness and intellectual disability (in Canada, a “dual diagnosis”) and who reside in shelters for the homeless are vulnerable and at risk for physical, sexual, and financial abuse. The same factors that make them vulnerable may result in neglect by the very services designed to help them. Their mental illnesses are difficult to diagnose and treat, and their intellectual impairments are difficult to ascertain. The authors review the existing literature on homelessness and intellectual disability, and use this and their extensive clinical experience with the homeless in a large Canadian city to identify and extrapolate reasons for the challenges facing both these individuals and the agencies and organizations that serve them. The authors identify several challenges to providing appropriate services, such as identifying and linking with these adults; recognizing the implications of the level of the intellectual delay and the mental illness on the individual’s ability to cope, and stay safe; understanding the wants and needs of the individual; expanding the mandate of the agencies and professionals wanting to serve this group; and developing policy that will both provide appropriate support of the individual, but also provide protection, if necessary by legal means. They further suggest that focal services should be based on the following characteristics: flexibility, meeting the client where he or she lives, consideration of capacity to consent in order to avoid neglect issues, and understanding of the lifelong and pervasive effects of intellectual delay in all facets of life. Though the population discussed is resident in Canada, these issues apply to individuals in other countries, as well, where there are concerns about these vulnerable adults with a “triple diagnosis.”