Mood disorders, also referred to as aﬀective disorders (from the Latin aﬀectus, meaning emotion or mood) are the commonest conditions encountered in psychiatry. It has been estimated that the life-time risk of being aﬄicted with one of them is in the order of 12 per cent for men and 20 per cent for women. Essentially, they are exaggerations of the universal human capacity to experience sadness and elation. Psychiatry classiﬁes these moods as illness when they are judged to be extreme, incapacitating, of long duration, disproportionate to the circumstances, and found to be unresponsive to advice or outside inﬂuence. The polar extremes of mood range from the profound misery of severe depression to the ecstatic self-conﬁdence and limitless energy of mania.