Nearly every cellular activity, such as cell growth, proliferation, protein synthesis, and nuclear division, is regulated by protein phosphorylation/dephosphorylation on tyrosine residues as initiation events.Synchronization occurs through interaction of protein tyrosine kinases and phosphatases. This book unveils signal pathways that connect extracellular signals with changes of phosphorylation state of target proteins allowing coordinated and integrated modulation of mitogenic signalling. Specifically, cascades involving protein serine/threonine kinases and connections to phosphoinositides derived second messengers are discussed.