The authors examine current challenges confronting psychiatry in caring for homeless people with psychiatric disorders. After reviewing how psychiatry has historically addressed homelessness and mental illness, the authors discuss the roles that the profession has developed in working with homeless populations. These roles, which encompass clinical, administrative, academic, and advocacy functions, have evolved as a result of trends both in homelessness services and within the profession of psychiatry. Challenges implicit in this evolution are discussed, including recent trends in homelessness, particularly an increase in prevalence, especially among families and children and some clinical subpopulations. The authors propose that these epidemiological trends are affecting the mental health care needs of homeless people. To be effective and credible in continuing to help solve the problems of homeless people with psychiatric disorders, psychiatry must adapt to these new challenges, using the roles it has developed.