Transcranial direct current stimulation (tDCS) is a noninvasive method to activate or de-activate neurons in superficial regions of the brain through the induction of weak electric currents in the brain tissue delivered by two scalp electrodes. Initial studies have shown tDCS to be effective for treating major depressive disorder (MDD), although there are negative trials in the specialized literature. One reason for these discrepant results might be that the duration of tDCS treatment in clinical trials to date (up to 2 weeks) is still insufficient to produce consistent clinical improvements. Thus, we intend to assess, in a sample of outpatients with MDD, whether a 3-week adjunctive course of active tDCS over the left dorsolateral prefrontal cortex is associated with a significant clinical improvement when compared to sham tDCS. The investigators hypothesize that subjects receiving active tDCS will present with significantly higher response and remission rates at weeks 3.