Metal-on-metal hip resurfacing arthroplasty has undergone a recent resurgence as an alternative treatment option for young and active patients with significant osteoarthritis. The claimed advantages of metal-on-metal hip resurfacing arthroplasty include lower wear rate, preservation of bone stock for subsequent revision procedures, restoration of anatomic hip mechanics, and enhanced stability due to the larger diameter of articulation. A disadvantage, however, is that the metal-on-metal resurfacing releases large amounts of very small wear particles and metal ions. The long-term biological consequences of the exposure to these Co-Cr particles and ions remain largely unknown. The purpose of this review is to provide an overview of the current literature on the adverse periprosthetic biological reactions associated with metal-on-metal hip resurfacing arthroplasty.