Major Depressive Disorder (MDD) represents an enormous mental health challenge. Despite successful medication and psychotherapies, fewer than half of patients achieve remission, and relapse is more likely in individuals who do not fully recover. For these reasons, efforts are focusing on the identification of vulnerability factors associated with the onset, maintenance, and relapse of depression. Meditation and other mental training exercises deriving from Buddhist and Hindu traditions represent one potentially fruitful extension to contemporary models of depression, as well as a complement to existing medication and psychotherapy treatments. This chapter provides a contemporary theoretical account of MDD as a bio-psychosocial condition that has been enriched by mindfulness principles. In doing so, we create linkage to the traditional cognitive behavioral model, which has always viewed MDD as arising from a failure to access metacognitive skills that promote healthy emotional processing. Recent efforts to emphasize the cultivation of metacognitive awareness instead of changing cognitive content coincide with the explosion of interest in mindfulness principles and practice. This chapter reviews findings from studies that include mindfulness-enriched treatments for MDD and other emotional disorders and then frames issues facing our field given the promising start in incorporating mindfulness principles into our models.