In the present study, the so called "black box" is revealed by testing the mechanisms that explain the Human Resource Development (HRD) and employee performance relationships, including organizational citizenship behavior. The literature proposes that the learning capacity of individuals, groups and organizations plays an important role in these relationships. Both, employees’ perceptions of HRD (perceived HRD) and human resource managers’ perception of HRD (intended HRD) are taken into account. The hypothetical model was tested with 223 employees and 35 HR managers from 30 small, medium- and large-size organizations in Germany and the Netherlands. The results showed that intended and perceived HRD were significantly related to each other. This correlation was significantly higher for small organizations. Intended HRD was significantly related to employee performance and this relationship was significantly mediated by individual learning capacity. Further, there is a significant relationship between perceived HRD and employee performance and this relationship is mediated by individual learning capacity while controlling for group- and organizational learning capacity.