Elevation of intracellular free Ca²⁺ is one of the key triggering signals for T-cell activation by antigen. A remarkable variety of Ca²⁺ signals in T cells, ranging from infrequent spikes to sustained oscillations and plateaus, derives from the interactions of multiple Ca²⁺ sources and sinks in the cell. Following engagement of the T cell receptor, intracellular channels (IP₃ and ryanodine receptors) release Ca²⁺ from intracellular stores, and by depleting the stores trigger prolonged Ca²⁺ influx through store-operated Ca²⁺ (CRAC) channels in the plasma membrane. The amplitude and dynamics of the Ca²⁺ signal are shaped by several mechanisms, including K⁺ channels and membrane potential, slow modulation of the plasma membrane Ca²⁺-ATPase, and mitochondria that buffer Ca²⁺ and prevent the inactivation of CRAC channels. Ca²⁺ signals have a number of downstream targets occurring on multiple time scales. At short times, Ca²⁺ signals help to stabilize contacts between T cells and antigen-presenting cells through changes in motility and cytoskeletal reorganization. Over periods of minutes to hours, the amplitude, duration, and kinetic signature of Ca²⁺ signals increase the efficiency and specificity of gene activation events. The complexity of Ca²⁺ signals contains a wealth of information that may help to instruct lymphocytes to choose between alternate fates in response to antigenic stimulation.