This chapter discusses the scope of principles of fiduciary duty as they appear in the canon law. It first provides a historical background on canon law and its relation to fiduciary law, noting that the medieval church and principles of fiduciary duty were interconnected in direct and positive ways. In fact, the church was governed by many of the same principles of fiduciary law that are found in modern trust law, and these principles were fully and authoritatively stated in the Corpus iuris canonici during the twelfth and thirteenth centuries. The chapter proceeds by analyzing the Corpus iuris canonici and its two books: Gratian’s Concordia discordantium canonum, also known as the Decretum, and the books of Decretals. It also traces the development of fiduciary law inherent in some of the canonical texts and explains how fiduciary principles came to be enforced in the canon law, citing examples of the width of the scope of fiduciary principles found in English court practice, including a duty applied only to the clergy. Finally, it considers whether the modern law of trusts was shaped in any way by canonical influence.