The online educational environment is increasingly being used by adults and should be designed based on the needs of adult learners. This article discusses andragogy, an important adult learning theory, and reviews three other adult learning theories: self-directed learning, experiential learning, and transformational learning. During this discussion, the theories are examined for the ways in which they may be applied to the design of online learning environments. In addition, the characteristics of adult learners are examined, and an analysis of how these characteristics influence the design of an online learning environment is presented. Recommendations follow regarding how to design an online classroom environment while considering the application of adult learning theories. (Contains 2 tables)