Signal transduction plays a pivotal role in cell regulation. Hormones, cytokines and neurotransmitters bind to their specific receptors in the plasma membranes of cells. The receptors of many hormones are coupled to G-protein coupled signalling pathways. Insulin and growth factors are coupled to receptor-linked kinase. The second messenger is generated in cells once the first messenger binds to the receptors of plasma membranes. Cyclic adenosine monophosphate (cyclic AMP), inositol 1,3,5-triphosphate (IP3) and calcium ion (Ca2+) have been demonstrated to play a role as a second messenger in cells. The pivotal role of Ca2+ in cellular regulation was established with the finding of calmodulin and protein kinase C that modulates the effect of Ca2+ in the regulation of cellular functions. Many receptor classes and their downstream kinases activate Erk, Akt, Stat, Smad, NF-κB and other cascades to regulate gene expression and cellular function. This book provides recent research in the study of the new developments in signalling transduction in cells.