Cell homeostasis is controlled and regulated by multiple signalling proteins that operate almost in all cellular compartments. Their common task is to process regulatory signals from both the extracellular and intracellular spaces by triggering a cascade of intracellular events leading to modulation of downstream gene activity. One of the important signalling pathways is represented by the STAT multigene family comprising seven members. In general, various STATs act as potent transcription factors delivering signals of diverse polypeptide ligands (ie cytokines and growth factors) into the nucleus. This review summarizes some up-to-date data on the role of STAT1 in maintaining cellular homeostasis with the emphasis on its role in the control of cell growth, proliferation, apoptosis, and immune reactions. Part of the review deals with expression and posttranslational abnormalities of this molecule identified in a variety of human pathological conditions including cancer. The direct or indirect involvement of STAT1 in the process of malignant transformation is highlighted in view of these molecular perturbances that may contribute to oncogenesis and that may be potentially used as novel targets for anticancer therapy.