This is the first collection of papers devoted to Ludwig Wittgenstein's cryptic but brilliant On Certainty. This work, Wittgenstein's last, extends the thinking of his earlier, better known writings, and in so doing, makes the most important contribution to epistemology since Kant's Critique of Pure Reason--a claim the essays in this volume help to demonstrate. The essays have been grouped under four headings, reflecting current approaches to the work: the Framework, Transcendental, Epistemic, and Therapeutic readings.