The Jak-STAT pathway is one of many pleiotropic signaling pathways that plays an important role in organismal development and in response to changing environmental cues. As a key signaling cascade for cytokines and growth factors, Jak-STAT plays central role in the innate and adaptive immune system. Cytokines control the stability, commitment, and maturation of cytotoxic and helper T cells, parts of the adaptive immune system that mediate immunity to pathogens and are linked to inflammatory diseases. Dysregulation of Jak-STAT protein expression or function leads to autoimmunity, allergic diseases, and cancer. Because of their central role in these responses, Jak and STAT molecules have been targeted to develop therapeutics. This review extensively discusses the mechanism of how Jak-STAT signaling in T cells defines our immune responses in the battle against foreign pathogens.