This chapter defines andragogy and reviews the early foundations of adult learning theory. Previous adult learning research performed with multiple constraints demonstrated that circumstance (education, training, health, speed of response) may have more of an impact in learning than age. Studies also revealed that age impacts the ability to perform some cognitive functions yet has little impact on others. While the characteristics of the adult learner have remained relatively consistent, perspectives in classifying the topic and its principles have varied. In discussion of self-directed learning, the authors address related objectives, ethos, self-directed attributes, and assessment methods. The authors report a decline in literature focused on self-directed learning within adult education and advocate for continued investigation and research. 8/10