The serine/threonine kinase glycogen synthase kinase-3 (GSK-3) was initially identified as a key regulator of insulin-dependent glycogen synthesis. GSK-3 was subsequently shown to function in a wide range of cellular processes including differentiation, growth, motility and apoptosis. Aberrant regulation of GSK-3 has been implicated in a range of human pathologies including Alzheimer's disease, non-insulin-dependent diabetes mellitus (NIDDM) and cancer. As a consequence, the regulation of GSK-3 and the therapeutic potential of GSK-3 inhibitors have become key areas of investigation. This review will focus on the mechanisms of GSK-3 regulation, with emphasis on modulation by upstream signals, control of substrate specificity and GSK-3 localisation. The details of these mechanisms will be discussed in the context of specific signalling pathways.