The spectrum of biological systems which makes use of the signal transducers and activators of transcription (STAT) paradigm extends beyond the interferon system in which it was first discovered to include many other cytokines and agonists. Having catalogued which STATs are activated by each stimulus, investigators have turned their attention to defining the biological processes and the genes regulated by the STAT pathway. These studies are in their early stages. Although many tools have been developed to probe the STAT pathway, e.g., mutant receptors, dominant-negative STATs, chemically dimerizable STATs, and mice lacking STAT proteins, more is known about the biological phenomenon affected than the molecular mechanism or the STAT-regulated genes involved. The cellular events currently believed to utilize STAT-dependent pathways can be grouped according to those which affect cell growth, differentiation, and apoptosis.