Unicompartmental knee arthroplasty (UKA) is designed for patients presenting arthritic wear limited to a single medial or lateral tibiofemoral compartment. The indication is based on strict criteria. Wear must stem from degenerative osteoarthritis or be secondary to aseptic necrosis of the medial condyle. Inflammatory rheumatism is a contraindication. Age and activity level should be compatible with an indication for arthroplasty. The body mass index should be less than 30kg/m². The ligament system must be intact, particularly both cruciate ligaments. Any pre-existing axis deformity should be moderate and the residual axis deformity, after correction of wear with a unicompartmental tibial augmentation spacer, should not exceed 7–10° varus or valgus. These highly restrictive conditions result in the ideal indications for UKA suitable for no more than 15–20% of knee arthroplasty candidates for most surgeons experienced in this procedure. Although the results of certain early series worried potential users, today it can be asserted that recent series whose indications and technique correspond to modern use criteria, have shown results that are as reliable as those of total knee arthroplasty (TKA) at a 10years’ follow-up. Beyond this time frame, the risk of polyethylene wear related to the technical restrictions of the UKA is another consideration. Indeed, to prevent the risk of rapid extension of osteoarthritis to the opposite compartment, the procedure should be limited to restoring the patient’s constitutional axis before wear phenomena had set in. This makes UKA a surgical procedure at risk of failure due to wear phenomena. Much of this paper will describe the precise rules for UKA positioning, which are critical to observe to warrant these implants outcome and longevity.