Mature T cells are produced in the thymus and released into the bloodstream in low numbers. These cells are considered to be immunologically naıve until such time as they encounter MHC-peptide complexes for which their T-cell receptors (TCR) have high affinity. Recognition of antigen in appropriate form, ie in association with costimulatory signals on the surface of professional antigen-presenting cells (APCs), leads to extensive T-cell proliferation and differentiation into effector cells. Once the infection has been cleared, it is no longer of benefit to the host to maintain high numbers of effector cells and most of the activated T cells die by apoptosis. However, a proportion of these cells survive, leaving the frequency of cells specific for the priming antigen much higher among memory T cells than that which existed among naıve T cells. This difference in frequency makes a major contribution to the nature of the secondary response, which is typically faster and of greater magnitude than the primary response. In addition, T cells may also carry a true ‘memory’of a prior response to antigen, exhibiting differences from naıve T cells at the single cell level. Here we provide a brief overview of the qualitative differences that have been reported to exist between naıve and memory T cells and evidence that memory T cells themselves are functionally heterogeneous.