The andragogical model of adult learning and education developed by Malcolm Knowles. the basis for much of "adult learning theory", is summarized and reviewed in terms of its assumptions, principles and recommended practices. By recasting the model as a theory with attendant hypotheses, it is then critiqued in terms of its theoretical adequacy and empirical support. Theoretically, the model is found wanting because it slights the full range of adult learning experiences, makes misleading distinctions between adult and child learners, minimizes individual differences between adult as learners, and does not adequately deal with the relationship between motivation and learning. Empirically, research testing the effects of andragogy provides inconclusive and contradictory outcomes. New directions for establishing a better theory of learning effectiveness are suggested. (Contains 3 tables and 7 endnotes.)