Cellular signaling forms the basis of physiological well-being of the cell. Reversible protein phosphorylation is one of the key mechanisms employed for cell signaling. Tyrosine residues when phosphorylated by specific signaling enzymes called the Protein Tyrosine Kinases (PTK) serve as the 'signaling currency' of the cell. To ensure that this currency is wisely, Protein Tyrosine Phosphatases (PTP) regulate their required amounts in the cell. The PTPs hence ensure a signaling-balance and homeostasis critical to cell survival and growth. The role of PTPs is well established in neuronal development, growth and differentiation, glucose homeostasis, immune response and also platelet signaling. PTPs are increasingly finding their position at the frontiers of drug discovery. This book will aim to describe all these advances in the field of PTP structure and signaling. The book will serve as an essential reference for anyone working on protein phosphorylation, signaling and drug discovery.