This article explores Immanuel Kant’s transcendental argument in philosophy. According to Kant, a transcendental argument begins with a compelling first premise about our thought, experience, knowledge, or practice, and then reasons to a conclusion that is a substantive and unobvious presupposition and necessary condition of the truth of this premise, or as he sometimes puts it, of the possibility of this premise’s being true. Transcendental arguments are typically directed against skepticism of some kind. For example, Kant’s Transcendental Deduction targets Humean skepticism about the applicability of a priori metaphysical concepts, and his Refutation of Idealism takes aim at skepticism about an external world. The article first considers the nature of transcendental arguments before analysing a number of specific transcendental arguments, including Kant’s Transcendental Deduction and Refutation of Idealism. It also discusses contemporary arguments, such as those forwarded by P. F. Strawson and and Christine Korsgaard, together with their problems and prospects.