The concept of abnormal mood has been a matter of a millennia-long debate in philosophy and medicine, while the diagnosis and classification of mood disorders remains a complex and controversial issue even in modern psychiatry. A centrepiece of this debate is the conceptualisation of mood and, by extension, mood disorders as a multi-dimensional spectrum with transdiagnostic symptoms (i.e., a continuous diagnostic classification) or as discrete nosological entities (i.e., a categorical diagnostic classification). Theoretical models and arguments based on empirical evidence have been proposed for both the distinct categorisation of abnormal mood states and the affective continuum perspective, which may also encompass psychosis and psychotic disorders. Although the conceptualisation of mood as a spectrum ranging from unipolar depression to unipolar mania may be the most suitable, this approach requires further evidence before it can replace the categorical classifications firmly employed in clinical practice for more than a century.