Written and edited by experts in the field, this collection from Cold Spring Harbor Perspectives in Biology reviews the arsenal of memory T-cell subtypes in the body and how they function as key components of the adaptive immune system. The contributors examine the heterogeneity and diversity of memory T-cell subtypes, how they are classified and named, and their biological characteristics--their transcriptional and metabolic features, migration patterns, life spans, and surveillance activities. The differentiation of T cells from a naive state, how they establish and maintain a primed memory state, and factors that contribute to their decline are all covered.