Transubstantiation is the doctrine of the Catholic Church which teaches that the substance of the bread in the sacrament of communion is changed into the substance of the Body of Christ (and the substance of the wine into the substance of the Blood of Christ). Nevertheless the “outward characteristics” of the bread and the wine (the “eucharistic species”) remain unaltered --- which means that “transubstantiation” is not the same as “transmaterializion.” The theological term “substance” is also critical to the Christian idea of the Trinity, where three distinct “persons” of God are said to be “consubstantial” as the One. John Hamer of Toronto Centre Place will look at the development of these theological terms and propositions and consider what Catholics and other Christians mean when they talk about the “Body of Christ.