Self-directed learning (SDL) is among the most productive areas of research in adult education. Malcolm S. Knowles is credited with a comprehensive synthesis of adult teaching and adult learning principles. Andragogy, the art and science of helping adults learn, lies at the heart of Knowles' work. Lucy M. Guglielmino theorized regarding the occurrence of self-direction in learning. Stephen D. Brookfield considered SDL from both cognitive and behavioral perspectives. Ralph G. Brockett and Roger Hiemstra embraced the view that the concept of SDL should be seen as only one part of a broader concept, and their conceptualization of SDL was a synthesis of other writers' multidimensional conceptualizations. According to Philip C. Candy, SDL involves the following tenets: (1) the interaction between a person and his or her environment; (2) knowledge as tentative, evanescent, and socially constructed; (3) learning as a qualitative shift in how phenomena are viewed; and (4) individuals as engaging in complex, mutually interdependent relationships with their environments. The research of these individuals has led to many theoretical approaches to SDL, including the following: humanism; personal responsibility orientation; behaviorism; neobehaviorism; critical perspectives; and constructivism. Numerous researchers have used qualitative and quantitative research methods to expand understanding of SDL. (Contains 147 references.) (MN)