Mood disorders are chronic, recurrent psychiatric disorders with high morbidity rates that cause severe disability. Researchers have used molecular imaging extensively in studies of mood disorders. In this article, we concisely and selectively review the major findings of positron emission tomography studies of patients with mood disorders. Specifically, we describe findings from cerebral blood flow, cerebral glucose/oxygen metabolism, and radioligand studies in both cross-sectional and longitudinal investigations. Patients with mood disorders have mood-correlated regional metabolism changes and molecular abnormalities in several neurotransmitter systems. Although the findings of these studies are not completely consistent and confounding factors, including drug effects and specific methodology, should be strictly controlled, these results reveal the pathophysiology of mood disorders and aid the development of novel treatment approaches for mood disorders. Future positron emission tomography research will benefit greatly from the development of better radioligands to simultaneously identify multiple neurotransmitter systems in the specific brain region and the integration of more detecting methods in specifying the neurobiological predictors of treatment response in patients with mood disorders. Understanding the molecular mechanisms in underlying mood disorders will result in aetiological diagnosis and individualization of treatment of these disorders.