This is a comprehensive and critical analysis of the philosophical foundations of adult education. Six major systems of educational thought -- liberal education, progressivism, behaviorism, humanistic education, radicalism, and conceptual analysis -- provide the framework for an examination of the views of prominent adult educators and major programs in adult education. This much-needed work contributes to our understanding of the personalities, issues, and programs in the field of education; it also stimulates readers to clarify their own personal philosophy of education. A bibliographic essay presents the developments in each of the six systems over the past 15 years. Three major areas -- phenomenology, critical theory, and feminist theory -- are discussed within this framework.