Signal transduction plays a pivotal role in cellular regulation. The second messenger is generated in cells once the first messenger binds to the receptors of plasma membranes. The second messenger, which was found firstly, is cyclic adenosine monophosphate. After that, intracellular calcium ion (Ca2+) has been demonstrated to play a role as a second messenger for hormonal stimulation in cells. The pivotal role of Ca2+ in cellular regulation was established with the finding of calmodulin and protein kinase C that modulate the effect of Ca2+ in the regulation of cellular functions. Calcium signalling is probably the most ubiquitous cellular signal that mediates the action of many hormones, cytokines, and neurotransmitters. This book provides an in depth examination of calcium signalling.