Finding an effective antidepressant agent for a patient is a process of trial and error. Improvement generally requires 4 to 12 weeks of treatment. Patients may stop taking medication too soon because of delayed response or intolerable side effects. Fewer than half of patients achieve full remission on the first drug that is tried. Predicting how a patient might respond to treatment would help identify an effective treatment more quickly, avoid unnecessary medication changes, and encourage treatment adherence. Electroencephalography (EEG) is an old technology used to measure brain waves and diagnose epilepsy. In this article, I describe the use of quantitative EEG (QEEG) for predicting antidepressant treatment response.