The central claim of this paper is that On Certainty (1969) is not to be seen as a "workâ€? of Wittgenstein, but as an assemblage from the enormous collection of material that constitutes the Nachlass (cf. Stern 1996; Ackermann 1998; Hintikka 1991). To substantiate this assertion, I will critically compare the publication to its sources, namely manuscripts 172, 174, 175, 176 and 177 of the Nachlass. I will trace and value the decisions G.E.M. Anscombe and G.H. von Wright have made in gathering this collection. Their claim is twofold: first, Wittgenstein "marked it [On Certainty] off in his notebooks as a separate topicâ€?, and second "it constitutes a single sustained treatment of the topicâ€? (Wittgenstein 1969, Preface). I will show that the first claim is incorrect: not only are Wittgenstein"s "marksâ€? ambiguous, but the editors applied their own demarcations in these notebooks as well. Their second claim is in need of amplification. Wittgenstein"s concern with epistemological issues is not limited to these five manuscripts: other notebooks of the same period also contain entries that strongly relate to topics discussed in On Certainty. I will discuss examples from the undated notebooks MSS 169, 170 and 171, published in Last Writings on the Philosophy of Psychology, Vol. II: The "Innerâ€? and the "Outerâ€? (1992). These show that it is difficult to make strict divisions in the Nachlass in general, and in the late writings of the years 1949-1951 in particular.