It has recently been argued that a unidimensional approach to mood disorders, focusing predominantly on emotions is too narrow to capture the full spectrum of symptoms of affective disorders, and that additional symptom domains of activity and cognition are at least equally important – possibly even more so . This ‘ACE’ (Activity, Cognition and Emotion) model provides more clarity in conceptualising mixed states, the existence of which challenges the predominantly dichotomous view promulgated by DSM and ICD, in which mania and depression are deemed to be mutually exclusive and form two poles of mood disorders with little in between. However, in reality, there are symptoms that occur in both depression and mania, and to better understand mixed mood states it has been proposed that these ‘overlapping’ symptoms form the essence of mixed states, and therefore they should be central to their definition. Furthermore, within these, overlapping symptoms of distractibility, irritability and psychomotor agitation are possibly the most important for better defining mixed states and advancing our understanding. Thus, in this brief article we endeavour to further refine our focus, specifically narrowing in on irritability, and argue that perhaps ‘it’ in particular holds the key to defining these complex presentations and unlocking their secrets.This article is protected by copyright. All rights reserved.