The JAK-STAT signal pathway refers to a cytokine-stimulated signal transduction pathway which plays an important role in numerous biological processes which include cell proliferation, differentiation, apoptosis and immune regulation. The signal transducer and activator of transcription (STAT) pathway is linked upstream with the Janus kinase (JAK) family of proteins. It is also capable of integrating inputs from several signaling pathways. Each member of the family performs a distinct role in signal transduction and is critical in mediating cellular responses to various types of cytokines. STAT family members particularly STAT3 and STAT5 have been associated with the progression of cancer. Persistent STAT3 and STAT5 activation is known to stimulate chronic inflammation, which makes healthy cells more susceptible to carcinogenesis. STAT3 is significantly linked to immune evasion and tumor growth. Several mechanisms appear to play a role in the activation of STAT5 proteins in human cancer, comprising miRNA- and hormone-regulated TFs, gene amplification, epigenetic alterations, aberrant growth factor signaling and altered proteolytic pathways. This book contains some path-breaking studies on the STAT pathway. It will serve as a valuable source of reference for graduate and postgraduate students.