Upon encountering antigen, T cells clonally expand and differentiate into effector cells that directly or indirectly eliminate antigen-bearing pathogens. When renewed contact with the same pathogen occurs the immune response is mounted in a faster and more accurate way, a process that is referred to as immunological memory. The basis for T-cell memory is at least partially provided by an enhanced precursor frequency of antigen-specific T cells, and an increased responsiveness of primed T cells to activation signals. In contrast to B cells, which acquire mutations in the immunoglobulin genes after antigenic challenge, somatic markers are lacking that distinguish unprimed (or naive) from primed (encompassing memory and effector) T cells. Instead, differential expression of cell surface molecules on subsets of T cells and measures for replicative history can be used to obtain insight into the antigen-driven development of the T-cell compartment. Apart from fundamental issues addressing lineage relationships between naive, memory and effector T cells and the cellular basis for long-term T-cell memory, these types of studies have proved to be valuable in understanding T-cell reconstitution in situations of severe T-cell depletion, i.e., after chemotherapy, treatment with depleting CD4 monoclonal antibodies or during HIV infection.