Due to the high population prevalence of major depression and the strong emphasis on pharmacotherapy for this disorder, antidepressants are among the most frequently prescribed pharmacological agents. But the clinicians are still unable to predict accurately the response of their depressed patients to medication. This article reviews the biological predictors of treatment response including monoamine, neuroendocrine, pharmacogenetic, and psychophysiologic markers. The biological predictors of response, despite some interesting leads that may in the long term be of considerable importance, are not yet sufficiently established to be of routine clinical usefulness. Many of the predictive factors explored in this article are examples of mediators and moderators that affect outcomes. Each one alone may not provide definitive answers for predicting response to treatment, but each must be taken into account at the outset of treatment. It is clear that treatments must be individualized for each patient. It would be necessary to develop the algorithm in order to predict the responsiveness of antidepressant treatment with integration of the results from the previous studies.