The problem and the solution. The field of human resource development (HRD) has been viewed as being supported by three theoretical foundations—namely, economic, psychological, and system theories. Although contributions of adult learning theory have been long acknowledged, it is more important to recognize its unique role of incorporating three theoretical foundations and consequently providing a distinct foundation of HRD. By identifying the relationship between HRD and one of its closely related fields, adult education, this article argues that it is adult learning theory that provides a foundation and linkage for both fields.