Generation of intracellular Ca2+ signals in response to Ca(2+)-mobilizing stimuli is a critical event in the control of many cellular processes. Inositol 1,4,5-trisphosphate (IP3) represents a dominant second messenger subserving the release of Ca2+ from intracellular store sites. The protein on the surface of which the IP3 receptor is located comprises an IP3-gated Ca2+ channel, and binding of IP3 to this receptor triggers the release of Ca2+ through this channel. The receptor for IP3 displays a close resemblance to the ryanodine receptor, another intracellular Ca2+ channel, in many molecular and physiological properties. Many lines of evidence strongly suggest the central role that the IP3 receptor plays in the conversion of numerous external stimuli to intracellular Ca2+ signals characterized by complex spatiotemporal patterns such as Ca2+ waves and oscillations. In this review, we shall summarize our current knowledge of the structure and function of the IP3 receptor in order to understand the way how the activity of this important receptor is regulated to accomodate itself to the generation of diverse intracellular Ca2+ signals.