To demonstrate the clinical effectiveness of an antidepressant drug requires evidence beyond short- and long-term efficacy, including a favourable adverse-effect profile and sustained treatment adherence. Under these conditions, patients should experience enhanced social and functional outcomes. The novel antidepressant agomelatine, a melatonergic MT(1)/MT(2) receptor agonist with serotonin 5-HT(2C) receptor antagonist activity, displays antidepressant efficacy with a favourable adverse-effect profile that is associated with good patient adherence. Specifically, agomelatine has demonstrated significant short-term (6-8 weeks) and sustained (6 months) antidepressant efficacy relative to placebo, as well as evidence of relapse prevention (up to 10 months). In head-to-head comparative studies with venlafaxine and sertraline, there was evidence of early (at 1-2 weeks) and sustained (at 6 months) advantages for agomelatine. In addition to evidence of early efficacy, agomelatine also restored disturbed sleep-wake patterns early in treatment. There was no evidence of antidepressant-induced sexual dysfunction, weight gain or discontinuation-emergent symptoms. Agomelatine has demonstrated a range of properties that suggest it could offer advantages over current treatments for major depressive disorder, although further comparative trials are still required, as is evidence from real-world clinical practice.