This article fills a theoretical gap by identifying an understudied subject area for human resource development (HRD) theory building, learning participation of HRD interventions in organizations. The topic has critical significance in current HRD practices, such as concerns on e-learning dropout rates and HRD measurement and evaluation. First, a comprehensive literature review and analysis are presented to identify the research gap in general adult education research and management or HRD-related literature in learning participation. A conceptual framework of HRD learning participation is then proposed, from cross-sectional and time-series perspectives, to describe the pattern, factors, structure, and their interrelationships in HRD learning participation, with a discussion of model constructs. The conceptual framework is then operationalized with mathematical operations to demonstrate how to empirically test the model. Finally, the applicability of the proposed theory and its implications for future HRD research are elaborated.