In this important new text, Keith Lehrer introduces students to the major traditional and contemporary accounts of knowing. Beginning with the accepted definition of knowledge as justified true belief, Lehrer explores the truth, belief and justification conditions on the way to a thorough examination of foundation theories of knowledge, externalism and naturalized epistemologies, internalism and modern coherence theories as well as recent reliabilist and causal theories. Lehrer gives all views careful examination and concludes that external factors must be matched by appropriate internal ones to yield knowledge. Readers of Professor Lehrer's earlier book Knowledge will want to know that this text adopts the framework of that classic text. But Theory of Knowledge is a completely rewritten and updated version of that book that has been simplified throughout for student use.