The diagnosis of peri-prosthetic bone and joint infections relies on converging information from clinical, laboratory and imaging assessments. Clinical findings alone may suffice: a sinus tract is a major criterion that establishes the diagnosis of infection. Identifying the causative organism is crucial and requires the early collection of high-quality samples from sites in contact with the prosthetic material. The bacteriological samples may be obtained by aspiration or open surgery. Imaging techniques have undergone remarkable improvements over the last two decades. Ultrasonography can be performed early and can be used to guide a needle biopsy if appropriate. Computed tomography or magnetic resonance imaging shows the extent of bone and/or soft-tissue involvement, provided effective artefact-suppression techniques are applied. Nuclear medicine methods have an undefined place in the diagnostic strategy and their possible role must be evaluated during a multidisciplinary discussion. The array of new laboratory methods introduced in recent years includes microbiological culture techniques, molecular biology tests, antigen and antibody assays and tests for immune markers in blood and/or joint fluid. When the first-line investigations fail to provide a definitive diagnosis, a multidisciplinary discussion at a referral centre for complex osteo-articular infections makes a major contribution to defining the subsequent diagnostic strategy. This lecture focusses on the following six questions: does the clinical assessment still have diagnostic relevance? What is the diagnostic contribution of imaging studies? Must the infection be documented pre-operatively and if so, how? Which microbiological techniques should be used? Which non-microbiological investigations help to diagnosis peri-prosthetic bone and joint infections? What role do referral centres for complex bone and joint infections play in the diagnostic strategy?