Theory building and theory-building research are starting to draw increased attention from HRD scholars. There is a growing recognition of the importance of theory building in maturing thought and practice in HRD. This article examines what is meant by good theory and theory-building research in HRD through a synthesis of the existing literature and a discussion of the core concepts, issues, and challenges associated with theory building in an applied field. It also highlights the importance of embracing multiparadigm research perspectives for stepping up to the task of theory building in the HRD profession. © 2000 by Jossey-Bass, A Publishing Unit of John Wiley & Sons, Inc.