Publisher Summary Once a T cell is activated, it proliferates and differentiates into effector cells that carry out actions designed to eliminate antigens. Memory T cells that mediate secondary immune responses are also produced. Both effector and memory T cells are activated at lower levels of T cell receptor (TCR) engagement and costimulatory signaling than are naive cells, and express different subsets of adhesion molecules. This chapter discusses how daughter cells differentiate into Th1, Th2, or CTL effector cells, and explains the actions that they take to protect the host during an immune response. The chapter then describes how the primary response concludes with the elimination of effector T cells and the generation of memory T cells. The chapter ends with a discussion of the activation and effector functions of memory T cells that underlie the secondary response. Overall, all the cellular components of an adaptive immune response and how naive cells are activated when sufficient numbers of TCRs or B cell receptors (BCRs) interact with sufficient avidity with an immunogenic epitope are covered here. Such responses constitute the recognition and removal of entities that are nonself, and that are critical for the elimination of pathogens.