ciples of textual criticism--borrowed from the fields of classics and medieval studies--have a valuable application for plagiarism investigations. Plagiarists share key features with medieval scribes who worked in scriptoriums and produced copies of manuscripts. Both kinds of copyists--scribes and plagiarists--engage in similar processes, and they commit distinctive copying errors. When committed by plagiarists, these copying errors have probative value for making determinations that a text is copied, and hence, unoriginal. To show the efficacy of the newly proposed techniques for proving plagiarism, case studies are drawn from philosophy, theology, and canon law.