Since Plato, knowledge has been taken to entail justified true belief. On this view of knowledge, Plato's question as to why knowledge is more valuable than true belief must be answered in part by explaining the value of justified belief. An attractive explanation is that justified belief is belief of a sort likely to be true - an externalist account. Opposed to externalism is the internalist view that justification must be accessible to the subject or constituted by the subject's epistemic perspective. This book explores the nature and value of knowledge and justified belief by examining the debate between externalism and internalism. The author argues against the popular view that internalism is historically dominant epistemology by examining closely the epistemological principles that underline the treatment of skepticism in Plato, the Academic and Pyrrhonian skeptics, Descartes and Hume. He then develops a sustained argument against many forms of internalism in favour of an externalist, reliabilist epistemology. His version of reliabilism, though strictly externalist, accommodates and explains the most durable intuitions alleged to support internalism.