The 'Gettier Problem' has shaped most of the fundamental debates in epistemology for more than fifty years. Before Edmund Gettier published his famous 1963 paper (reprinted in this volume), it was generally presumed that knowledge was equivalent to true belief supported by adequate evidence. Gettier presented a powerful challenge to that presumption. This led to the development and refinement of many prominent epistemological theories: internalism, externalism, evidentialism, reliabilism, and virtue epistemology. The debate about the appropriate use of intuition as providing evidence in all areas of philosophy began as a debate about the epistemic status of the "Gettier intuition". The differing accounts of epistemic luck are all rooted in responses to the Gettier Problem. The discussions about the role of false beliefs in the production of knowledge are directly traceable to Gettier's paper, as are the debates between fallibilists and infallibilists. The "knowledge first" view was, in large part, provoked by the supposed failure of all solutions to the Gettier Problem. Indeed, it is fair to say that providing a satisfactory response to the Gettier Problem has become a litmus test of any adequate account of knowledge - even those accounts that hold that the Gettier Problem rests on mistakes of various sorts. This volume presents a collective examination by twenty-six experts, including some of the most influential philosophers of our time, of the various issues that arise from Gettier's challenge to the analysis of knowledge. Explaining Knowledge sets the agenda for future work on the central problem of epistemology.