Many agonists bring about their effects on cellular functions through a rise in cytosolic [Ca2+] ([Ca2+]c) mediated by the second messenger inositol 1,4,5-trisphosphate (IP3). Imaging studies of single cells have demonstrated that [Ca2+]c signals display cell specific spatiotemporal organization that is established by coordinated activation of IP3 receptor Ca2+ channels. Evidence emerges that cytosolic calcium signals elicited by activation of the IP3 receptors are efficiently transmitted to the mitochondria. An important function of mitochondrial calcium signals is to activate the Ca2+-sensitive mitochondrial dehydrogenases, and thereby to meet demands for increased energy in stimulated cells. Activation of the permeability transition pore (PTP) by mitochondrial calcium signals may also be involved in the control of cell death. Furthermore, mitochondrial Ca2+ transport appears to modulate the spatiotemporal organization of [Ca2+]c responses evoked by IP3 and so mitochondria may be important in cytosolic calcium signaling as well. This paper summarizes recent research to elucidate the mechanisms and significance of IP3-dependent mitochondrial calcium signaling.