A primary calcium (Ca2+) entry pathway into non-excitable cells is through the store-operated Ca2+ release
activated Ca2+ (CRAC) channel. Ca2+ entry into cells is responsible for the initiation of diverse signalling
cascades that affect essential cellular processes like gene regulation, cell growth and death, secretion and gene
transcription. Upon depletion of intracellular Ca2+ stores within the endoplasmic reticulum (ER), the CRAC
channel opens to refill depleted stores. The two key limiting molecular players of the CRAC channel are the
stromal interaction molecule (STIM1) embedded in the ER-membrane and Orai1, residing in the plasma membrane
(PM), respectively. Together, they form a highly Ca2+ selective ion channel complex. STIM1 senses the
Ca2+ content of the ER and confers Ca2+ store-depletion into the opening of Orai1 channels in the PM for
triggering Ca2+-dependent gene transcription, T-cell activation or mast cell degranulation. The interplay of Orai
and STIM proteins in the CRAC channel signalling cascade has been the main focus of research for more than
twelve years. This chapter focuses on current knowledge and main experimental advances in the understanding
of Orai1 activation by STIM1, thereby portraying key mechanistic steps in the CRAC channel signalling cascade.