Calcium is an essential messenger during T cell activation, proliferation and cytotoxicity. ORAI1 and STIM1 proteins represent the key components of calcium influx in T helper cells. In addition, ORAI1 and STIM1 mutations have been shown to impair cytotoxic T lymphocyte function in patients with severe combined immunodeficiency, thus indicating a major role for CRAC (calcium release activated calcium) channels in calcium homeostasis in CD8⁺ T cells as well. By using ratiometric calcium imaging experiments, RNAi and qRT-PCR, we showed that ORAI1 and STIM1 are expressed in primary human CD8⁺ T cells and we investigated their role in calcium signaling following TCR (T cell receptor) activation by antibody-coated beads or store depletion by thapsigargin. Gene silencing of ORAI1 reduces store operated calcium entry significantly in primary human CD8⁺ T cells. However, calcium signaling following TCR stimulation is less reduced by ORAI1 knockdown in naïve T cells compared to effector T cells. Additional STIM1 knockdown reduced calcium influx following TCR activation in naïve T cells significantly compared to ORAI1 silencing alone, while no potentiation of the effect of sole ORAI1 knockdown could be observed in effector T cells. This indicates differences in the activation of signaling pathways following store depletion and TCR activation. In addition, these results indicate that STIM1 may be involved in additional calcium entry routes independent of ORAI1 in naïve T cells.