Glycogen synthase kinase-3 (GSK3) has recently been linked to mood disorders and schizophrenia, and the neurotransmitter systems and therapeutic treatments associated with these diseases. GSK3 is a widely influential enzyme that is capable of phosphorylating, and thereby regulating, over forty known substrates. Four mechanisms regulating GSK3 (phosphorylation, protein complexes, localization, and substrate phosphorylation) combine to provide substrate-specific regulation of the actions of GSK3. Several intracellular signaling cascades converge on GSK3 to modulate its activity, and several neurotransmitter systems also regulate GSK3, including serotonergic, dopaminergic, cholinergic, and glutamatergic systems. Because of changes in these neurotransmitter systems and the actions of therapeutic drugs, GSK3 has been linked to the mood disorders, bipolar disorder and depression, and to schizophrenia. Inhibition of GSK3 may be an important therapeutic target of mood stabilizers, and regulation of GSK3 may be involved in the therapeutic effects of other drugs used in psychiatry. Dysregulated GSK3 in bipolar disorder, depression, and schizophrenia could have multiple effects that could impair neural plasticity, such as modulation of neuronal architecture, neurogenesis, gene expression, and the ability of neurons to respond to stressful, potentially lethal, conditions. In part because of these key actions of GSK3 and its associations with mood disorders and schizophrenia, much research is currently being devoted to identifying new selective inhibitors of GSK3.