The thesis concentrates on the role and status of the goddess Isis in one particular province of the Roman Empire, Achaia, in the first to third centuries A.D. The literary sources, particularly Plutarch’s work De Iside et Osiride, Apuleius’ Metamorphoses, and the Greek novels, are examined in great detail, especially in relation to current philosophy of the period. While other works have explored the popularity of Isis in the wider Roman Empire, my thesis focuses upon the feelings of the Greeks and the reasons for their acceptance of the goddess. Although I focus upon the appeal of Isis as a saviour goddess, this aspect of her popularity should be placed firmly alongside others, and should not be considered her most significant quality to the detriment of others. Other reasons for Isis’ popularity in Roman Greece include her manifestation as Tyche, her use as a philosophical principle, her status as a moral exemplar, and her role as a vehicle for displaying wealth, all of which are significant for the study of ancient religion and, in particular, knowledge of the mystery religions in Greece at a date later than the Hellenistic period. By attempting to understand why Isis appealed to Greeks of the Roman Empire, my thesis reveals much that is of value about the social life of Greece under Roman rule, and about how religion, philosophy, and daily life became inseparably intertwined. Both Plutarch and Apuleius demonstrate that the cult had an intellectual appeal, which has not often been explored or proposed as a reason in its own right for Isis’ popularity. The Isis of the novels is a beneficent goddess, whose good moral qualities the protagonists may assimilate. As Tyche, Isis plays a role at the beginning and end of novels, framing their events. Through a study of the Isis aretalogies, the Hermetic Corpus, and the Greek Magical Papyri, it can be seen that Isis appealed to Greeks of all social classes, and that reasons for the goddess’ popularity remained constant regardless of a person’s social class. This was because philosophical ideas filtered down to the lower classes, and because the festivals and exotic rites that attracted the lower classes had an equal appeal to educated Greek elites. The philosophical ideas of the cult are reflected in its architecture, grave stelae, and the theophoric names of Greek citizens, which use ‘Isis’ as an element. The Isis cult acquired a social prestige, philosophical interpretation, and immense universal popularity that is important for the study of the status of religion in Greece, and the status of Greece in the wider Roman Empire.