The National Institutes of Health, Centers for Disease Control and Prevention, and World Health Organization jointly sponsored a workshop on protective and disease-enhancing immune responses to respiratory syncytial virus (RSV). The primary purpose of the meeting was to discuss protective and disease-enhancing immune responses to RSV in the context of opportunities and barriers to the development of RSV vaccines. Although both live attenuated and subunit vaccines have been developed, it is not yet clear if any of these vaccines will be safe and effective. The fact that neither the disease-enhancing nor the protective immune response to RSV is well understood or well characterized is an important barrier to development of these vaccines. Studies in animal model systems and newly developed immunologic tools, however, provide hope that these barriers can be overcome and a safe and effective RSV vaccine can be developed.