This chapter offers an exploration of the nature and principles of ancient Graeco-Roman disease classification. It does so with a particular focus on mental or psychological disorders, though this analysis will be contextualised within the larger picture of Graeco-Roman medical disease classification and its methodology, procedures and aims. Particular attention will be paid to Galen of Pergamon (second century CE) - our richest medical source and also a particularly interesting locus for study of the interface between medical and philosophical approaches - but this author will be considered in close comparison with our other major medical sources for mental disorders in the Roman imperial period, in particular Aulus Cornelius Celsus (first century CE), Aretaeus of Cappadocia (first-second century CE) and Caelius Aurelianus (fifth century CE), and also contextualised through an overview of broader historical developments from the classical (ca. fifth century BCE) to the late antique period (ca. fifth to seventh century CE).