This volume, number 43 of the collection Auctores Britannici Medii Aevi, is the sixth of the project of editing the Dialogus of William of Ockham, initiated by John Kilcullen and John Scott more than 30 years ago at the invitation of David Luscombe. Volumes 20 (published in 2011: Dialogus: Part 2. Part 3, Tract 1, ed. John Kilcullen, John Scott, Jan Ballweg, Volker Leppin), 33 (2019: Dialogus: Part 3, Tract 2, ed. Semih Heinen, Karl Ubl), 35 (2020: Dialogus: Part 1, Books 1-5, ed. John Kilcullen, John Scott), 41 (2023: Dialogus: Part 1, Book 6, ed. George Knysh), and 42 (2023: Dialogus: Part 1, Book 7, ed. George Knysh) give access to the critical edition of William of Ockham’s treatise, while the present volume offers an English translation of the Latin text published in volume 35, i.e., books 1-5 of Part 1, On Heretics, as well as the briefer chapters 5-16 from the treatise Against John, edited by H.S. Offler and published in volume III of Ockham’s Opera Politica in 1956.