An overview is offered of Wittgenstein's groundbreaking discussion of knowledge and certainty, especially in his final notebooks, published as On Certainty. The main interpretative readings of On Certainty are discussed, especially a non-propositional/non-epistemic interpretation and a variety of propositional and/or epistemic interpretations. Surveys are offered of the readings of On Certainty presented by such figures as Annalisa Coliva, John Greco, Danièle Moyal-Sharrock, Duncan Pritchard, Genia Schönbaumsfeld, P. F. Strawson, MichaelWilliams, and CrispinWright. This Element demonstrates how On Certainty has been especially groundbreaking for epistemology with regard to its treatment of the problem of radical scepticism.