HR practices with high quality contents do not necessarily result in positive individual and organisational outcomes due to shortcomings in their implementation. This intended-implemented gap has resulted in the shift from content-focused to process-focused perspective in addressing the HRM black box which has brought the concepts of HRM implementation and its effectiveness to the fore. While more attention has been paid to effective implementation of HR practices and the topic of HRM implementation in recent years, there is still a lack of a systematic understanding of how and why implementation of HR practices succeeds or fails. As such, to unfold the HRM black box, a comprehensive and exhaustive review of the mechanisms through which effective implementation is achieved, the contextual factors that influence the operation of such mechanisms, and the outcomes that such mechanisms produce is required. In this study, a Realist Synthesis is conducted to identify and synthesise existing evidence from the empirical studies of HRM implementation into Context-Mechanism-Outcome (CMO) configurations of each involved actor. These CMO configurations enhance the understanding of ‘what works for whom in which circumstances?’ regarding HRM implementation. As such, this Realist Synthesis clarifies what is dealt with in HRM implementation and contributes to designing, executing, and evaluating future HR practices and their implementations.

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