Evaluation of a patient with a failed total knee arthroplasty begins with a detailed history of the index knee arthroplasty and with the patient's medical history. The nature of the complaint after arthroplasty can help determine the etiology of failure. The primary causes of failure of total knee arthroplasty are pain, postoperative stiffness, and instability. Pain associated with weight bearing is most often mechanical and is caused by loosening, component failure, or patellar dysfunction. Continuous pain can be associated with infection or complex regional pain syndrome. Persistent postoperative stiffness may be caused by inadequate rehabilitation or improper balancing of flexion and extension spaces. However, loss of motion after satisfactory mobility has been achieved may be associated with infection, synovitis, tendinitis, or component loosening. Instability after total knee arthroplasty results from improper balancing, inappropriate component size, and component failure. Posterior instability generally occurs during flexion. Medial-lateral instability can result from either improper balancing of components or incompetent collateral ligaments. Radiographs can detect loosening and osteolysis, as well as component wear, fracture, and malposition. Nuclear scans can aid in detecting loosening and infection. If infection is suspected, aspiration is mandatory to attempt to confirm the diagnosis and identify an organism.