This study aims to systematically synthesize more than 20 years of human resource outsourcing (HRO) studies in a way that is meaningful, concise and, useful to HRO researchers and practitioners. Using grounded coding technique blended with integrative literature review, empirical papers published from 1997 to 2018 are examined. This is the first HRO literature review using such techniques to answer three key research questions: What has the existing empirical academic literature revealed about the determinants of HRO decisions and outcomes? What are the key issues emerged from the literature? What are the gaps in knowledge which warrant future HRO research? Other than providing a comprehensive insight into an emerging research area, this review also contributes to laying an important foundation for the initial descriptive HRO theory development. 36 dependent variables and 99 independent variables over 449 relationships were coded.