We know of the importance of HRM implementation for the effectiveness of HRM and the crucial role that line managers play in this process. Underlying some of this research, there has been the implicit assumption that effective implementation occurs when line managers faithfully and consistently translate corporate policies in the ways they are intended. While this idea is not inherently wrong, it tends to pay little attention to line managers’ agency in that process, and to the need for workforce differentiation. It also runs against recent conceptualizations of implementation, which explicitly acknowledge its malleable nature, emphasizing variability in the way in which different actors directly or indirectly influence how a particular policy is used. Line managers may not only implement HRM in a narrow sense, but also complement, edit, and innovate in delivering HRM to their work units, modifying their HRM-related decisions as they see fit. More research is needed that considers line managers’ HRM implementation from different perspectives, acknowledging their agency and contribution to HRM differentiation. In this symposium, we will aim to find answers to the following questions: What is the role and agency of line managers in HRM implementation? And in what way is line managers’ HRM differentiation related to effective HRM implementation?