Abstract This chapter discusses Galen’s ideas concerning human ailments, their classification and etiology, and how they relate to entities from earlier medical traditions. It shows that Galen has set the scene for later developments by combining two approaches to disease: the classical notion of imbalance and the popular idea of functional damage. First, it outlines the status of pathology in Galen’s project and presents the context of its development. Next, the various concepts involved in it are defined. The structure of Galen’s nosology is then described and its meaning examined. Finally, it provides an account of the multiple levels of causation in play and how they inform the diagnostic procedure.