Compelling evidence suggests that inflammation contributes to the development of depression. Many depressed individuals have higher levels of proinflammatory mediators, which appear to interact with many of the pathophysiological domains of depression, including neuroendocrine function, neurotransmitter metabolism, and synaptic plasticity. This is further supported by observation that therapeutic administration of interferon-α (IFN-α) leads to depression in a significant proportion of patients. These findings suggest that targeting proinflammatory cytokines and their signaling pathways may represent a unique therapeutic opportunity to treat depression and related conditions, such as labile anger, irritability, and fatigue.