 In the Critique of Pure Reason Kant argues that space and time are merely formal features of how we perceive objects, not things in themselves that exist independently of us, or properties or relations among them. Objects in space and time are said to be “appearances”, and he argues that we know nothing of substance about the things in themselves of which they are appearances. Kant calls this doctrine (or set of doctrines) “transcendental idealism”, and ever since the publication of the first edition of the Critique of Pure Reason in 1781, Kant’s readers have wondered, and debated, what exactly transcendental idealism is, and have developed quite different interpretations. Some, including many of Kant’s contemporaries, interpret transcendental idealism as essentially a form of phenomenalism, similar in some respects to that of Berkeley, while others think that it is not a metaphysical or ontological theory at all. There is probably no major interpretive question in Kant’s philosophy on which there is so little consensus. This entry provides an introduction to the most important Kantian texts, as well as the interpretive and philosophical issues surrounding them.