Although phosphorylation of proteins on tyrosine is relatively rare compared to phosphorylation on serine or threonine residues, the past two decades of research into PTP function have led to a great appreciation of the critical role PTPs have in regulating basic cellular processes. Among these important roles is the regulation of cellular signaling pathways related to metabolism. This volume contains chapters which highlight many aspects of PTP function in the context of metabolism. Given the growing obesity and diabetes epidemics in the United States and throughout the world, the desire to identify possible therapeutic targets for treatment of these diseases is a high priority. In many ways, PTPs may be attractive drug targets since they are amenable to targeting with small molecules; however many challenges abound in making PTP inhibitors.