Despite increasing interest in human resource management (HRM) implementation as an explanation for the association between HRM and firm performance, considerable confusion remains about what implementation means. In order to develop conceptual definitions of HRM implementation and implementation effectiveness, this study builds on three different literatures outside the HRM field (strategy, innovation, and change management), which have addressed this topic extensively. As a result, implementation is characterized as a dynamic process, involving the interaction among multiple actors, starting with the adoption of a new practice and ending with its routinization. This is distinguished from implementation effectiveness as an outcome of that process. The study helps to achieve construct clarity, hence providing a more solid basis for future research and allowing for a better consolidation of findings. The authors also develop an agenda for further research by reviewing a number of theoretical and methodological approaches that have been used in implementation research across fields, including HRM. Overall, the study aims to establish implementation research as a highly relevant academic and practical quest not only in HRM, but also in other management literatures.