Periprosthetic joint infection (PJI) is one of the most challenging and frequent complications after lower-extremity joint (hip and knee) arthroplasty. However, there is no single accepted set of diagnostic criteria for PJI. Various definitions have been proposed; however, none have been widely adopted. Furthermore, some of these definitions disagree with each other . Therefore, a workgroup convened by the Musculoskeletal Infection Society (MSIS) analyzed the available evidence to propose a new definition for PJI. A summary of recommendations of those in attendance at a premeeting workshop of the 21st Annual Meeting of the MSIS on August 4, 2011, pertaining to the definition of PJI is outlined below. Existing published data on the definition of PJI was discussed by e-mail in the preceding 6 months by the executive members of the MSIS and a group of experts with known interest in this field. The intention of this proposal is to have a “gold standard” definition for PJI that can be universally adopted by all physicians, surveillance authorities (including the Centers for Disease Control, medical and surgical journals, the medicolegal community), and all involved in management of PJI. The panel acknowledged, in certain low-grade infections (ie, Propionibacterium acnes), several of these criteria may not be routinely met despite the presence of PJI. Using this definition, clinicians can be confident in their diagnosis and therefore provide appropriate treatment. Additionally, adoption of this definition for research purposes will allow for consistency between studies and potential improvement of the quality of the published body of evidence.