On B and T lymphocytes, ligation of the antigen receptor (AgR) induces a biphasic Ca2+ response. In the initial phase there is a large elevation in the intracellular Ca2+ concentration as a consequence of Ca2+ release from intracellular stores. This is followed by a lower, but prolonged elevation that is dependent on extracellular Ca2+. 1, 2 This simple description belies the complexity of the response. The initial phase may involve as many as three different intracellular Ca2+ channels, while the second phase depends not only on plasma membrane Ca2+ channels, but also on at least two different intracellular channels. The complexity of the signal, and the many opportunities for regulation of individual components of the signalling mechanism, lead to a tremendous flexibility in outcome, ranging from single, brief elevated Ca2+ transients, through a range of oscillatory responses, each of which can be decoded by the cell into a differing outcome. 2 In this review we concentrate on the Ca2+ channels involved in the AgR-mediated Ca2+ signal, but we briefly discuss other Ca2+ channels present in lymphocytes. Figure 1 shows two possible schemes for the involvement of Ca2+ channels in TCR signalling, and Fig. 2 shows possible roles for Ca2+ channels in B cells.