In this paper, we emphasize the importance of focusing on internal fit, external fit, and employee cognitive frameworks in HRM systems. We observe that cumulative research streams in the SHRM literature have not proven, or simply have assumed the existence of the synergetic effect of HRM practices, and that works examining the condition of its functioning have been few. We argue for the condition of synergetic effect of HRM practices, which has to incorporate ideas of internal fit, external fit, and the focus on employees' cognitive mechanisms. We also argue that future research would be more fruitful with emphasis on examining the systems effect of HRM system of the firm.