This chapter provides an overview of the current literature on human resource management outsourcing (HRO). Human resource management outsourcing involves contracting out activities traditionally performed by the organization’s human resource department to an outside organization. While HRO is a popular topic, there are few reliable sources on the extent to which organizations implement the approach. Further, research has indicated that the overall impact of HRO is not always straightforward, with studies suggesting small positive effects on company performance and mixed effects on outcomes related to the functioning of human resource departments. This chapter examines common theoretical foundations of HRO, the need to differentiate outsourcing of different types of human resource activities, factors influencing organizations’ decisions to outsource, and effects of these decisions. It presents empirical data from a comparative HRM study on the prevalence of outsourcing for insights into the contested question of how widespread HRO really is and suggests future directions for HRO research.