Glycogen synthase kinase‐3 (GSK‐3) regulates multiple cellular processes, and its dysregulation is implicated in the pathogenesis of diverse diseases. In this paper we will focus on the dysfunction of GSK‐3 in Alzheimer’s disease and Parkinson’s disease. Specifically, GSK‐3 is known to interact with tau, β‐amyloid (Aβ), and α‐synuclein, and as such may be crucially involved in both diseases. Aβ production, for example, is regulated by GSK‐3, and its toxicity is mediated by GSK‐induced tau phosphorylation and degeneration. α‐synuclein is a substrate for GSK‐3 and GSK‐3 inhibition protects against Parkinsonian toxins. Lithium, a GSK‐3 inhibitor, has also been shown to affect tau, Aβ, and α‐synuclein in cell culture, and transgenic animal models. Thus, understanding the role of GSK‐3 in neurodegenerative diseases will enhance our understanding of the basic mechanisms underlying the pathogenesis of these disorders and also facilitate the identification of new therapeutic avenues.