Glycogen synthase kinase-3 (GSK-3) is a pivotal molecule in the development of Alzheimer's disease (AD). GSK-3beta is involved in the formation of paired helical filament (PHF)-tau, which is an integral component of the neurofibrillary tangle (NFT) deposits that disrupt neuronal function, and a marker of neurodegeneration in AD. GSK-3beta has exactly the same oligonucleotide sequence as tau-protein kinase I (TPKI), which was first purified from the microtubule fraction of bovine brain. Initially, we discovered that GSK-3beta was involved in amyloid-beta (Abeta)-induced neuronal death in rat hippocampal cultures. In the present review, we discuss our initial in vitro results and additional investigations showing that Abeta activates GSK-3beta through impairment of phosphatidylinositol-3 (PI3)/Akt signaling; that Abeta-activated GSK-3beta induces hyperphosphorylation of tau, NFT formation, neuronal death, and synaptic loss (all found in the AD brain); that GSK-3beta can induce memory deficits in vivo; and that inhibition of GSK-3alpha (an isoform of GSK-3beta) reduces Abeta production. These combined results strongly suggest that GSK-3 activation is a critical step in brain aging and the cascade of detrimental events in AD, preceding both the NFT and neuronal death pathways. Therefore, therapeutics targeted to inhibiting GSK-3 may be beneficial in the treatment of this devastating disease.