Primary total knee arthroplasty is the treatment for end-stage arthritis of the knee; in the last years, it is becoming more common and reliable, due to technical and implant improvement. With larger implant rates, the overall complications will increase and pain is the most common sign of implant failure. Pain can be related to a lot of different clinical findings, and the surgeon has to be aware of the various etiologies that can lead to failure. Pain does not always mean revision, and the patient has to be fully evaluated to have a correct diagnosis; if surgery is performed for the wrong reason, this will surely lead to a failure. In this paper, the authors revised the more common causes of failure that can have a painful onset proposing an approach for diagnosis and treatment.