Evaluation of the painful knee arthroplasty requires a careful history and examination with judicious use of laboratory tests, aspiration of joint fluid for synovial fluid analysis and culture testing, and occasional use of radionuclide imaging tests. The investigative process requires differentiation of the potential extrinsic and intrinsic causes of knee pain. Classification of intrinsic sources of knee pain into aseptic loosening, infection, instability, impingement, wear debris, and miscellaneous categories facilitates the evaluation process. Use of an algorithm to proceed through the process of selecting appropriate testing modalities is helpful. Newer technologies hold promise for differentiating certain entities such as infection or polyethylene wear.