This chapter discusses electrophysiological predictors of treatment response to antidepressants. Abnormalities of EEG alpha power and asymmetry have been found in depressed patients. Although there have been conflicting findings, alpha asymmetry abnormalities may represent biological traits associated with risk for developing a depressive disorder. Patients who respond to an antidepressant with a specific mode of action, e.g. an SSRI, may share a common biological substrate not present in nonresponders. Relative theta power was lower in treatment responders than in nonresponders and predicted response with an accuracy of 63%. The most promising EEG predictors are resting alpha and theta with eyes closed. Specific measures derived from the EEG, e.g. current source density, cordance, low resolution electromagnetic tomography analysis (LORETA), antidepressant treatment response (ATR) index, have all shown value as predictors, but further research is needed to compare their physiological interpretability and relative merits.