The term 'mood stabilizer' has been applied to a number of medications for the treatment of patients with bipolar disorder. The operational definition of the properties of a mood-stabilizing medication has varied according to the properties of specific medications and the clinical characteristics of the illness. Randomized controlled trials of agents accepted or proposed as mood stabilizers are reviewed to marshall the available evidence in support of this claim. In addition, potential pharmacological mechanisms underlying mood-stabilizing effects of established compounds are reviewed.