Painful total knee arthroplasty (TKA) represents a diagnostic challenge for the clinician and radiologist, as there is a wide variety of potential etiologies, with a broad range of clinical presentations, and the abnormalities on imaging studies are often subtle, absent, or nonspecific. Imaging findings of normal TKA are reviewed, in addition to a variety of complications such as loosening, infection, instability, osteolysis, heterotopic ossification, extensor mechanism disruption, and fracture. Although imaging evaluation of painful TKA is usually limited to conventional radiographs and nuclear imaging, examples of the utility of computed tomography are also illustrated, and suggested imaging strategies and algorithms are discussed.