Results from pivotal registration trials in major depressive disorder cohere with outcomes from effectiveness studies indicating that the majority of individuals receiving single-agent pharmacotherapy fail to achieve and sustain symptomatic remission. Several factors provided the impetus for this review: suboptimal efficacy with existing pharmacotherapy for major depressive disorder, quetiapine XR efficacy in the acute and maintenance treatment of bipolar depression, emerging pharmacodynamic evidence that quetiapine XR (and/or its metabolites) uniquely engages monoaminergic systems salient to symptom relief in depressive syndromes, the increasing use of second-generation antipsychotics in the treatment of major depressive disorder and the recent FDA review of quetiapine XR in major depressive disorder. Studies reviewed herein are pivotal registration trials that evaluated the acute and maintenance efficacy and tolerability of quetiapine XR (as monotherapy and as adjunctive treatment) in major depressive disorder. In addition, we also review recent investigations characterizing the pharmacodynamic effect of quetiapine's principal active metabolite, norquetiapine. All studies were obtained from AstraZeneca (Wilmington, DE, USA) and have been presented at national/international scientific meetings. Taken together, extant studies demonstrated that quetiapine XR (50 - 300 mg) provides rapid and sustained symptomatic improvement in the acute and maintenance treatment of major depressive disorder. Quetiapine XR may also offer advantages relative to duloxetine in time to onset of antidepressant action. The major limitations of quetiapine XR use in major depressive disorder relate to weight gain and disrupted glucose/lipid homeostasis as well as sedation/somnolence. Quetiapine XR has tolerability advantages compared with duloxetine on measures of sexual dysfunction. The data from the studies reviewed herein also indicate that quetiapine XR poses a low risk for extrapyramidal side effects in middle-aged and elderly individuals with major depressive disorder.