This document provides an overview of andragogy, the adult learning theory. It defines andragogy as the art and science of adult learning. The term was coined in 1833 and was popularized by Malcolm Knowles in the 1970s. Knowles identified five assumptions of adult learners that differ from the assumptions about child learners, including that adult learners are self-directed, draw on life experiences, are problem-centered, and are internally motivated. Knowles also described four principles of andragogy, such as involving adult learners in planning instruction and making the instruction problem-centered and immediately relevant. The document reviews these concepts and provides examples to explain their application to adult education.