 Callicles and Thrasymachus are the two great exemplars in philosophy of contemptuous challenge to conventional morality. Both are characters in Platonic dialogues, in the Gorgias and Book I of the Republic respectively; both denounce the virtue of justice, dikaiosunê, as an artificial brake on self-interest, a fraud to be seen through by intelligent people. Together, Thrasymachus and Callicles have fallen into the folk mythology of moral philosophy as ‘the immoralist’ (or ‘amoralist’). These are perhaps not quite the right words, but it is useful to have a label for their common  challenge—more generally, for the figure who demands a good reason to abide by moral constraints, and denies, implicitly or explicitly, that this demand can be  met.[1]  Because of this shared agenda, and because Socrates’ refutation of Callicles can be read as an unsatisfying rehearsal for the Republic, it is tempting to assume that the two share a single philosophical position. But in fact Callicles and Thrasymachus are by no means interchangeable; and the differences between them are important both for the interpretation of Plato and philosophically, for our understanding of the varieties of immoralism and the motivations behind it. This article discusses both the common challenge presented by these two figures and the features which separate them, treating them strictly as players in Plato’s philosophical dramas. (Thrasymachus was a real person, a famous diplomat and orator of whose real views we know only a little; of Callicles we know nothing, and he may even be Plato’s  invention.)[2]  It will also compare them to a third Platonic version of the immoralist challenge, the one presented by Glaucon and Adeimantus in Republic Book II, and to the writings of sophist  Antiphon—the best-known real-life counterpart of all three Platonic views, and perhaps their historical original. Certain aspects of Plato’s own arguments against immoralism will also be discussed, insofar as they help to clarify what Callicles and Thrasymachus themselves have to say.