 This article has traced the meaning of transcendental idealism, sometimes referred to as “critical” or “formal” idealism, through the text of the Critique of Pure Reason and various interpretive controversies. Historically, the main question dividing different interpretations is whether Kant is a phenomenalist about object in space and time and, if so, in what sense. The phenomenalist interpretation of Kant, dominant among Kant’s immediate predecessors and later German idealists, was challenged in twentieth century Anglophone scholarship by, among others, Graham Bird, Gerold Prauss, and Henry Allison. Some later scholars have retained a central idea of these scholars’ reading—that the appearance/thing in itself distinction is a distinction between distinct aspects of objects, not distinct kinds of objects—while jettisoning the purely epistemological interpretation of Kant’s idealism. The meaning and philosophical significance of “transcendental idealism” has been debated by Kant’s readers since 1781, and this debate shows no sign of abating any time soon.