Adult education as a discipline in the United States is a relative newcomer compared to other areas of education, other fields of practice such as medicine and law, and other disciplines. In fact, whether it can be considered a discipline at all is still debated in the U.S. Field of practice, applied discipline, practical discipline, or emerging discipline are more common terms used to describe the field of adult education. This paper traces the development of the knowledge base of adult education in the U.S. since the early decades of the twentieth century to the present. A major theme throughout its development is the tension between drawing from other disciplines versus developing a unique, indigenous body of knowledge. The final section of the paper addresses how social, professional, and academic forces are shaping the future development of the field of adult education.