Pharmacology constitutes a huge and significantly under-researched part of Galen’s work, representing roughly one-seventh of his enormous written output. It also accounts for a major part of his significance and influence in later centuries, in many periods and socio-medical contexts and through a wide variety of pathways of transmission. The aim of the present chapter is threefold. First, it gives a theoretical overview, looking at the nature and scope of Galen’s pharmacological texts and at the theory that they expound, placing this theory in relation to the previous and contemporary pharmacological tradition. Secondly, it considers Galen’s pharmaceutical practice, exemplified by his use of both “simple drugs” and recipes. Finally, it explores the significance and influence of his pharmacological work in later centuries, focusing on the particular example of the early Arabo-Islamic tradition.