Receptor-evoked Ca²⁺ signalling involves Ca²⁺ release from the endoplasmic reticulum, followed by Ca²⁺ influx across the plasma membrane. Ca²⁺ influx is essential for many cellular functions, from secretion to transcription, and is mediated by Ca²⁺-release activated Ca²⁺ (I_crac) channels and store-operated calcium entry (SOC) channels. Although the molecular identity and regulation of I_crac and SOC channels have not been precisely determined, notable recent findings are the identification of STIM1, which has been indicated to regulate SOC and I_crac channels by functioning as an endoplasmic reticulum Ca²⁺ sensor^,,,, and ORAI1 (ref. ) or CRACM1 (ref. ) — both of which may function as I_crac channels or as an I_crac subunit. How STIM1 activates the Ca²⁺ influx channels and whether STIM1 contributes to the channel pore remains unknown. Here, we identify the structural features that are essential for STIM1-dependent activation of SOC and I_crac channels, and demonstrate that they are identical to those involved in the binding and activation of TRPC1. Notably, the cytosolic carboxyl terminus of STIM1 is sufficient to activate SOC, I_crac and TRPC1 channels even when native STIM1 is depleted by small interfering RNA. Activity of STIM1 requires an ERM domain, which mediates the selective binding of STIM1 to TRPC1, 2 and 4, but not to TRPC3, 6 or 7, and a cationic lysine-rich region, which is essential for gating of TRPC1. Deletion of either region in the constitutively active STIM1^D76A yields dominant-negative mutants that block native SOC channels, expressed TRPC1 in HEK293 cells and I_crac in Jurkat cells. These observations implicate STIM1 as a key regulator of activity rather than a channel component, and reveal similar regulation of SOC, I_crac and TRPC channel activation by STIM1.