The treatise On Theriac to Piso has been attributed to Galen since ancient times; the attribution is however disputed. This thesis argues that although the content and style of the treatise is heavily influenced by Galen its author differs on a wide range of issues from the beliefs of Galen expressed in undoubtedly authentic works on matters of pharmacology, philosophical doctrine, the history of Pergamum and the interpretation of Hippocrates to the extent that it should no longer be attributed to him. The thesis also attempts to establish the best possible text from the Greek, Latin and Arabic manuscript sources and to provide a clear English translation and a commentary on the text focussing in particular on matters relevant to the question of the authorship of the treatise.