Section II discusses various aspects of naive and memory T cell biology, including clonal expansion, reprogramming of genes including those encoding cytokines and cytotoxic granules, changes in the expression of cell surface proteins involved in cell-cell adhesion, homing of naive and memory T cells, the role of MHC and cytokines in the maintenance of naive and memory T cells, and the characterization and differentiation of virus-specific memory T cell heterogeneity in mice and humans. Novel methods of visualization of immune cells and immune systems are reviewed in Section III.