The heading of 'Irritable Mood' was introduced into the Index Medicus in 1985 and it is noted that this term is frequently used but seldom defined. It is therefore important that a consensus is reached concerning its definition in the context of psychopathology and a definition is offered. It seems important to distinguish irritable mood and its outward manifestations from violently assaultive behaviour although there may be a continuum of disturbance from mildly to severely disordered behaviour. Irritability has so far not attracted much attention from clinicians and yet it is a common mood associated with many psychiatric and non-psychiatric conditions. Severe irritability may cause considerable distress both to the patient, his associates and sometimes also those attempting to treat or otherwise help him. It may overshadow other symptoms and the correct diagnosis and management may be missed. The validity and reliability of a convenient self-assessment scale is re-examined in the light of further data. Data from previous studies in the context of mood disorders suggests that irritability is a mood state which is independent of the other major moods of depression and anxiety; the severity of irritability probably has an inverse correlation with age. The most important associated factors are listed in order to inform researchers of the possible methodological pitfalls.