This paper summarises the development of pharmacology in the Graeco-Roman world. The number of medicinal plants recorded almost trebled between 400 BC (the Hippocratic writings) and AD 250 as the Greeks and Romans discovered more about the regions beyond the Mediterranean. The transfer of medicinal substances was almost entirely from East to West, and from South to North with Alexandria and, later, Rome as the major entrepôts. Two Greek writers were of great importance because they were translated into Syriac, Arabic, and Latin, among other languages, and set the agenda for subsequent work in pharmacology. The descriptions of medicinal substances by Pedanius Dioscorides (fl. AD 67) created a model that others followed down to the 17th century. Galen theorized about the way in which drugs worked, and his suggestion that one might correlate illnesses and drugs in terms of four degrees of intensity (a suggestion never followed through systematically by Galen himself) led in the Middle Ages to the classification and investigation of drugs in these terms.