The events involved in T cell activation are initiated at the cell surface by the interaction of ligands with specific cell surface receptors on the T cell. Central to antigen-induced activation is the CD3/Ti complex, a complex multi-chain receptor responsible for antigen/MHC recognition and signal transduction. Triggering the CD3/Ti complex results in the generation of intracellular second messengers, IP3 and DG, which are derived from PI metabolism. The second messengers lead to increases in [Ca2+]i and activation of pkC, events causally linked to various cellular responses, including the production of IL-2 through as yet poorly defined pathways. Little is known about how other cell surface molecules that may provide an accessory function participate in such events. However, future genetic and biochemical studies are likely to shed light upon the mechanisms of signal transduction by the CD3/Ti complex and accessory molecules and the details of the intracellular events involved in the activation of a host of cellular genes associated with activation.