PTEN (Phosphatase and Tensin Homolog) is a dual-specificity phosphatase, and a member of the broad protein tyrosine phosphatase family. PTEN gene encode a phosphatidylinositol-3, 4, 5-trisphosphate 3-phosphatase. It plays an important role in a variety of cellular process including cell proliferation, cell differentiation, cell migration and cell death. When the PTEN enzyme functions properly, it acts as part of a pathway that signals cells to stop dividing and can cause cells to undergo programmed cell death when necessary. These functions prevent uncontrolled cell growth. When the PTEN enzyme function is disrupted, it can lead to human diseases, such as cancer, Cowden syndrome, chronic inflammatory syndromes, developmental defect and diabetes. Especially in cancer, PTEN is one of the most widely studied tumour suppresser genes. It has been found to be the second most frequently mutated gene in human cancers, second only to the tumour suppresser p53 gene. The goal of this book is to provide a wide-ranging forum in the PTEN research field, in particular, with a focus on the structure, mechanism of action and role in cell signalling and regulation. The authors contributing to this book, are all most active researchers in PTEN research field. They presented the most recent advances, approaches and future prospects on PTEN research. This book is suitable as both a professional reference and as a text for biological, medical, biomedical and pharmaceutical interest to scientists, clinicians, teachers and students.