The literature on adult learners is reviewed, and two models of adult learning are developed. Demographic, social, and technological trends that stimulate the increasing demand for learning opportunities are examined, and the views of those who see dangers in new pressures on adults to participate in organized learning activities are considered. The research findings about the characteristics of adult learners are reviewed, and profiles of adults participating in the three common forms of adult learning (self-directed, organized instruction, and degree-credit) are developed to answer questions about who participates in adult learning. Why they participate, and why not, is addressed through a synthesis of research about motivations and deterrents to adult learning. Various motivational theories are described, and what is known about motivation through research and theory is integrated into an explanatory model of the motivation behind adult participation in learning activities. The following additional areas of research are reviewed: learning processes as a function of aging, adult stages of development, phases of the life cycle, and preferences and practices of adult learners regarding such issues as subject matter, teaching methods, and scheduling options. A conceptual framework for analyzing the interaction between learners and their environments, which considers physiological, social, and psychological dimensions, is also presented. Appendices illustrate the range of thought about lifelong learning and list agreements among scholars on the basic concepts underlying the ideal of lifelong learning. A bibliography is included. (SW)