This thesis is an analysis of the De Graecorum hodie quorundam opinationibus (1645), a letter written by the Greek Catholic Leo Allatios to his friend, the papal doctor Paulus Zacchias. It provides an assessment of his reliability as a source for popular religion in seventeenth-century Greece. The text, which was written to an Italian doctor, is placed in its western context and the influence of Catholic western trends in the fields of antiquarianism, ecumenism and medicine on his interpretation and presentation of the material is considered in order to assess the reason for his interest in these practices and the depth of his knowledge of the subject matter. Each chapter considers his interpretation of a particular popular belief and assesses the belief in its own context and that of formal Orthodoxy in order to ascertain Allatio’s understanding of it. The first section of the thesis examines Allatio’s comments on the child stealing demon, the gello. The first chapter considers how the beliefs surrounding it relate to popular and Orthodox practice. This moves on to an examination of his comments on the relationship between this creature and baptism, assessing the extent to which he takes a western perspective on the matter. The third chapter once again concentrates in the gello, this time in relation to marriage, and asks why Allatios fails to consider one of the richest areas of popular practice. The second section deals with Allatio’s comments on the vrikolakas and compares his understanding of the creature both with that presented in popular beliefs and that of the Orthodox church. The final section considers Allatio’s attitude towards medicine in the text which was written to a western doctor: chapter seven note how unction is made to play a positive role in a text mainly concerned with the superstitions of the Greeks and the extent to which this affects Allatio’s treatment of the subject; chapter eight examines his scientific outlook and the influence of his Neoplatonism on the structure of the final section of the letter. Finally, the conclusion notes the influence of Allatio’s western viewpoint with regard to his Catholicism, ecumenism and his scientific understanding of the world and stresses the relationship which exists between popular and formal Orthodoxy.