During the 2003 severe acute respiratory syndrome (SARS) outbreak in Toronto, the potential introduction of SARS into the homeless population was a serious concern. Although no homeless individual in Toronto contracted SARS, the outbreak highlighted the need to develop an outbreak preparedness plan that accounts for unique issues related to homeless people. We conducted key informant interviews with homeless service providers and public health officials (n = 17) and identified challenges specific to the homeless population in the areas of communication, infection control, isolation and quarantine, and resource allocation. Planning for future outbreaks should take into account the need to (1) develop systems that enable rapid two-way communication between public health officials and homeless service providers, (2) ensure that homeless service providers have access to infection control supplies and staff training, (3) prepare for possible homeless shelter closures due to staff shortages or high attack rates among clients, and (4) plan for where and how clinically ill homeless individuals will be isolated and treated. The Toronto SARS experience provided insights that are relevant to response planning for future outbreaks in cities with substantial numbers of homeless individuals.