The metabotropic glutamate receptor 1 (mGluR1) is highly expressed in cerebellar Purkinje cells and is crucial for cerebellar function. Glutamate binding to mGluR1 activates Ca²⁺ release from endoplasmic reticulum (ER) Ca²⁺ stores and a slow excitatory postsynaptic current (sEPSC). We have previously demonstrated that the sEPSC is mediated by the canonical transient receptor potential (TRPC) cation channel TRPC3. TRPC3-deficient mice completely lack the mGluR1-sEPSC and inward currents evoked by the specific mGluR-agonist DHPG. By using two-photon Ca²⁺ imaging we explored mGluR1-dependent synaptic Ca²⁺ signals in Purkinje cells. We found that Ca²⁺ signals in dendritic shafts were not affected by the absence of TRPC3. The spine Ca²⁺ signals, however, were reduced and, in addition, their onset after parallel fiber stimulation was delayed in TRPC3-deficient mice. The second messenger linking mGluRs to TRPC3 is unknown. Moreover, the mechanism by which the ER Ca²⁺ content is maintained is still a matter of debate for central neurons, including Purkinje cells. In non-excitable cells TRPC channels have been shown to interact with the stromal interaction molecule 1 (STIM1) and Orai proteins that together are responsible for the replenishment of Ca²⁺ stores. Using a quantitative single cell RT-PCR approach and immunohistochemistry, we now demonstrated the presence of STIM1-2 and Orai1-3 in Purkinje cells. When analyzing newly generated Purkinje cell-specific STIM1-deficient (STIM1^pko) mice, we found distinct alterations in their motor coordination when walking on an elevated beam. We next studied synaptic signaling at parallel fiber-Purkinje cell synapses in STIM1^pko mice by using whole-cell recordings and confocal Ca²⁺ imaging in acute cerebellar slices. We found that both signals downstream of mGluR1 (Ca²⁺ release and sEPSC) are largely abolished in the absence of STIM1. Similarly, Ca²⁺ signals and inward currents evoked by the mGluR-specific agonist DHPG were largely absent in STIM1^pko mice. Ca²⁺ signals evoked by local dendritic application of caffeine, an agonist of ryanodine receptors in the ER membrane, were decreased in the absence of STIM1, indicating reduced Ca²⁺ filling of ER stores in the absence of STIM1. Thus, our results demonstrate that STIM1 represents a critical link between synaptic activation of mGluR1 and opening of TRPC3 channels. Independently of TRPC3 activation, STIM1 is involved in the regulation of the Ca²⁺ content of ER Ca²⁺ stores. TRPC3 is present in dendritic spines and together with STIM1 is essential for synaptically evoked Ca²⁺ signaling by providing a Ca²⁺ entry pathway downstream of mGluR1 activation that is not required for Ca²⁺ store replenishment.