Several lines of evidence support the existence of an underlying abnormality in brain energy metabolism may play a key role in the biology of mood disorders. The current study utilizes two distinct but complementary imaging techniques, fluorodeoxyglucose (FDG) positron emission tomography (PET) and multinuclear magnetic resonance spectroscopy (MRS), to better understand the nature of these metabolic abnormalities in major depressive disorder (MDD). The investigators hypothesize that individuals with depression will have increased metabolic activity as measured by PET in certain brain regions involved in mood regulation, but that this metabolic activity will be inefficient based on MRS findings. For this study, the investigators will study 10 medication-free, currently depressed participants with recurrent MDD, 10 depressed participants with recurrent MDD currently taking antidepressant medication, and up to 20 healthy control participants matched to depressed participants for age and gender. Depressed and healthy participants will each undergo one PET scan and one MRS scanning session.