One of the most common causes for revision surgery following total hip arthroplasty (THA) is dislocation. Dislocation is associated with a considerable amount of suffering and risks for the patient, and extra costs for the health care system. Compared with degenerative arthritis, the dislocation rate is doubled for avascular necrosis and multiplied by three times for congenital dislocation, four for fracture, five for nonunion, malunion or a failed hip arthroplasty, and eleven times after surgery for prosthetic instability. In analysing instability the cause may be assessed as 1) locally caused within the hip with explanatory radiographic findings, 2) locally caused without explanatory radiographic findings or 3) non-locally caused, i.e. non-compliant patient, neuromuscular or cognitive disorders. Revision strategies for instability are typically directed to correct the underlying aetiology, but also to strive for an upsizing of the head and liner.Cite this article: Ullmark G. The unstable total hip arthroplasty. EFORT Open Rev 2016;1:83-88. DOI: 10.1302/2058-5241.1.000022.