Compartmental arthritis forms a significant subgroup of patients presenting with osteoarthritis of the knee. Involvement of the medial compartment is the most common and can be successfully treated by unicompartmetal arthroplasty. The first step is to obtain an accurate diagnosis of the process of the disease, and, particularly, to evaluate the integrity of the ACL. Assessment requires clinical, radiographic and perhaps arthroscopic evaluation, but the final decision will depend upon judgement taken at the time of the arthrotomy. This improves with experience and it is essential that the surgeon has sufficient basic understanding and regular practice. The decision on the design of the prosthesis is critical. This should be based on ten-year survivorship studies with a success rate of at least 85% to 90%. The technically more demanding prostheses can give excellent long-term results in some centres. The less complex prostheses can produce good long-term results in more general use. Most studies consistently report lower complication rates, more rapid recovery and long-term results of better quality than TKR. Set against this must be the recognition that failure because of progression of the disease can occur in addition to that due to mechanical causes. There is no convincing evidence that the well-tried and tested prostheses have a significantly greater overall rate of failure than TKR. Reports of revision show that this is not technically demanding and is considerably simpler than for failed TKR with results which are probably little different from those of a primary arthroplasty. The resurgent interest in compartmental arthroplasty is based on encouraging reports from those few centres which have pioneered the treatment. Successful results require a detailed knowledge of the pathology, indications, technique and management of the procedure. This philosophy adds several extra dimensions to the routine of established knee arthroplasty.