The Critique of Pure Reason by Immanuel Kant (1724-1804) is without question one of the landmarks of the entire history of Western philosophy, comparable in its importance and influence to only a handful of other works such as Plato's Republic, Aristotle's organon of logical works, and Descartes's Meditations on First Philosophy. The Critique was first published in 1781, after a decade of intensive preparation, and within a few years became the center of attention in German philosophy, and shortly after that in other European countries with advanced philosophical culture such as Britain and France as well. In the hope of clarifying some of the obscurity of the work and forestalling its misinterpretation, Kant issued a substantially revised edition of the work in 1787, in spite of his extensive agenda of other philosophical projects. That only intensified the debate about Kant's position, and ever since, students and scholars of Kant's philosophy have had to study the composite work that is the product of those two editions of the Critique. The present Companion is designed to orient readers to the complex structure and arguments of the Critique, to the philosophical context within which it arose, and to the enormous influence it has had and continues to have on the subsequent history of philosophy.