Prosthetic joint infections (PJI) can be difficult to diagnose. Studies have shown that we are missing many infections, possibly due to poor diagnostic workup and the presence of culture-negative infection. PJI diagnosis requires a methodical approach and a standardised set of criteria. Multiple PJI definitions have been published with improved accuracy in recent years. The new European Bone and Joint Infection Society definition offers some advantages in clinical practice. It identifies more clinically important infections and accurately defines those with the highest risk of treatment failure. It reduces the number of patients with uncertain diagnoses. Classification of PJIs may offer a better understanding of treatment outcomes and risk factors for failure.