Abstract: Despite the remarkable continuity, over the centuries, of the Catholic Church's condemnation of Freemasonry and the clarity of her rationale for doing so, the current canonical discipline of Catholic-Masonic issues is the subject of considerable confusion.The canonical prohibition of Catholic membership of a Masonic Lodge, or society, was expressly articulated in canon 2335 of the 1917 Code of Canon Law, which attached a penalty of excommunication, latae sententiae. Further canonical effects explicitly linked to Masonry were contained in six additional canons spread throughout the Code. The 1983 Code of Canon Law contains no explicit mention of Freemasonry. Canon 1374 provides for indeterminate penalties for those who joins societies which "plot against the Church", but there is no consensus of what the canonical definition of plotting (machinationem) means, nor which societies, if any, might be intended by the canon. This dissertation seeks, through historical analysis of the origins of Freemasonry itself, and the Church's teaching against it, to correctly place Freemasonry, specifically membership of a Masonic society by a Catholic, within the penal law of the 1983 Code. Chapter I traces the origins of Freemasonry and the Church's opposition to it, through to the codification of the 1917 Code of Canon Law. Chapter II is a parenthetical consideration of the particular phenomena of American Freemasonry, which is often held out to be somehow less noxious than the often explicitly anti-clerical European variety, and demonstrates its peculiar, but no less damnable, nature. Chapter III is an examination of the 1917 Code of Canon Law. It considers the canons on associations generally, and various condemned societies in particular, and extrapolates the significance of the canonical context of the Code's treatment of Masonry as, variously, a crime against the faith and against authority. The chapter also offers a treatment of some basic principles of penal law, including imputability and the nature of crime and punishment in canon law. Chapter IV traces the canonical prohibition of Masonic membership by a Catholic through the process of reform and revision which resulted in the 1983 Code of Canon Law. It then examines the various scholarly commentaries on the subject, as well as how Masonry has been canonically treated under the ius vigens. Chapter V advances the argument that a Catholic joining the Freemasons can, in fact, commit two delicts by the same action: the delict of joining a prohibited society (c. 1374); and the delict of heresy (c. 1364). Masonic texts and rites of initiation are examined as possibly containing heretical material which a Mason explicitly embraces. The chapter finishes by establishing the existence, necessity, and justice of an enduring universal canonical prohibition of Catholic membership of the Freemasons.