OBJECTIVE To examine the progress of positron emission tomography (PET) as a tool for understanding the psychobiology of mood disorders, particularly major depression and bipolar disorder. METHODS Review of the literature on functional imaging of mood disorders. RESULTS Functional imaging techniques have been used in psychiatric research as a noninvasive method to study the behaviour and function of the brain. Techniques used so far have involved the manipulation of emotion in healthy volunteers, the evaluation of depressed (unipolar and bipolar as well as secondary depression), manic, and normal subjects under resting and various activation conditions, such as cognitive activation, acute pharmacological challenge, and chronic thymoleptic treatments. As a result, functional imaging studies tend to support abnormalities in specific frontal and limbic regions. CONCLUSIONS Different PET methods demonstrate consistent abnormalities in the prefrontal, cingulate, and amygdala regions. These findings are in agreement with past animal and clinical anatomical correlates of mood and emotions.