Since the discovery of their physiological roles in cytokine signalling, the Janus kinases (JAKs) and the signal transducers and activators of transcription (STATs) have attracted considerable attention, to the point that the concept of a intracellular signalling pathway, named JAK/STAT, has emerged. As originally defined, this pathway involves ligand-dependent activation of a particular class of receptor-associated tyrosine kinases, the JAK proteins, which phosphorylate themselves and receptor components, creating recruitment sites for STAT transcription factors. The STATs are phosphorylated, they dissociate from the receptor x JAK complex and translocate to the nucleus where they participate in transcriptional gene activation. Although this pathway was found initially to be activated by interferons, it is now known that a large number of cytokines, growth factors and hormonal factors activate JAK and/or STAT proteins. Recent findings have suggested that the interdependence of JAKs and STATs might not be absolute as originally thought.