In the early 1960s, European adult educators developed a new theoretical model of adult learning called "andragogy." In 1968, Malcolm Knowles introduced the concept of andragogy into U.S. adult education literature and identified four primary assumptions about adult learners: adults become increasingly independent and self-directing; they accumulate experience that becomes a resource for learning; they orient their formal and informal learning around their social and work roles; and they orient their learning toward performance rather than subject. From the early 1970s through the late 1980s, Knowles and other experts in the field of adult education initiated and advanced assumptions, theories, principles, and practices related specifically to characteristics of adult learners and advanced the theory that andragogical theories and principles were applicable to learners of all ages, depending on individual learners' developmental levels and learning situations. Although many authors published in the 1990s support these earlier works, other authors have expressed concerns about the direction of the fields of adult education and learning theory, including language particular to the field, the lack of an acceptable general theory of adult learning, and the need for additional research to validate prior positions and expand knowledge about certain aspects of andragogy. (Contains 38 references.) (MN)