Immunological memory provides excellent protection against a wide variety of different pathogens. Compared to the primary encounter with antigen, the secondary response is more rapid and effective. So much so, that reexposure often goes unnoticed by the host. The specific nature of memory implies that it is especially dependent on T and B cells, the cells of the immune system which recognize antigen; it follows that the changes which occur in these cells during priming must account in large part for the memory response. Here, we will summarize what is and what is not known about memory in the CD4 T-cell population.