Truthfulness is, so to speak, Kant’s go-to duty. He invokes it in a wide range of philosophical settings, such as his discussion of free will in the Critique of Pure Reason, in his argument for a pure moral theory in the Groundwork, in the detailed moral philosophy of the Metaphysics of Morals and in his late lectures on education. Even though its scope and its theoretical foundation vary, the duty not to lie remains Kant’s prime example of a strict and unequivocal obligation. By way of introduction, this chapter first provides a survey of some important passages in which Kant invokes or argues for the duty of truthfulness before turning to the textbook example that is the bone of contention between him and Benjamin Constant and presenting some reactions provoked by the main thesis of Kant’s essay “On a Supposed Right to Lie”: that there is an unconditional, absolute duty to be truthful even in emergencies.