Originally described as the signal-transducing pathway of interferons, the JAK-STAT pathway soon turned out to participate in the signalling of numerous other immune and even non-immune mediators. Several murine knockout models have been described that underline the biological significance of this signalling system. Some human diseases (mainly neoplastic) are also known where malfunctioning of the JAK-STAT pathway is considered to participate in the pathogenesis. In this brief review article we will try to make a synopsis of its biological and clinical significance.