The in vivo study of receptor binding potential in the human brain is made possible by positron emission tomography (PET) imaging. Here we review PET studies of neuroreceptor function in mood disorders - specifically, major depressive disorder (MDD) and bipolar disorder (BD). We concentrate on the most widely studied receptors of the serotonergic and dopaminergic systems. Specifically, the serotonin 1A (5-HT(1A)), serotonin 2A (5-HT(2A)), serotonin 1B (5-HT(1B)), dopamine 1 (D1), and dopamine 2/3 (D2/3) receptors. We also review PET studies of the serotonin transporter (5-HTT), the dopamine transporter (DAT), monoamine oxidase A (MAO-A), and the muscarinic 2 receptor (M2). On the basis of the PET literature as well as supporting genetic studies, postmortem data, and preclinical models of depression, and several models of how monoaminergic function is altered in mood disorders are discussed with respect to inflammation, endocrine dysfunction, depression subtypes, and altered neurocircuitry.