This book demonstrates that the principles 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.