The discussion concentrates on new insights that are being gained into the nature of both acute and memory virus-specific CD8+ T cell responses as a consequence of recent advances in technology. Contrasts are drawn between the situation for a readily eliminated RNA virus and a persistent DNA virus. The major determinant of long-term memory seems to be the nature of the events occurring during the acute, antigen driven phase of the host response. Consideration is given to the possible role of (...) persistent antigen, and to the idea of immune exhaustion. (shrink)