Transcranial direct current stimulation (tDCS) is a novel treatment approach for depression that has shown promising efficacy in four recent double-blind, randomized, sham-controlled trials (RCT) and a meta-analysis. This study is a RCT of tDCS in depressed patients, testing its efficacy in both unipolar and bipolar depression. Mood, cognitive test performance and biomarkers will be measured during the trial.