Total knee arthroplasty is an effective and satisfactory way of treating severe gonarthrosis. Instability is the second most common complication of total knee arthroplasty, following infection, in first five years of the procedure. Patients mostly complain about a functional pain and undulating mild swelling around the joint. The most dramatic scene is dislocation of the prosthesis; which is a very rare condition. Finding the underlying cause of the instability requires a detailed history taking and patient evaluation. Weight bearing plain radiographies and dynamic radiological procedures like stress radiographs are mostly needed to diagnose the instability. Treatment is mostly by the revision of prosthesis whereas difficulty in diagnosis of the main mechanism under the instability yields up to 30% insisting instability following revision surgeries. Thus, surgeon should be particular about soft tissue balance, symmetrical bone cuts, choosing appropriate implant type in index and revision procedures and must perform instability tests intraoperatively with trials and definitive components.