A central focus of work in epistemology over the past twenty-five years has been the debate between internalism and externalism. At issue is the very form of an epistemological theory, and with it, competing conceptions of the epistemological enterprise. Internalists hold that the factors which make a belief justified must in some sense be internal to the agent: for example, they must be cognitively or introspectively available. Externalists deny this, holding instead that the features which make a belief justified may include such things as the reliability of the process by which the belief is formed, features which need not be available to the agent. This book brings together the essays which have defined and advanced this debate. It includes work by William P. Alston, Laurence BonJour, Earl Conee and Richard Feldman, Richard Foley, Alvin Goldman, Hilary Kornblith, Frederick Schmitt, Ernest Sosa, and Barry Stroud.