Observations during the past 35 years on operative and conservative treatment of adult and young patients with isthmic spondylolisthesis, demonstrate the main indications of fusion are pain and risk of further progression of the slip. At one time, posterior fusion was recommended. Posterolateral fusion in situ was adopted in 1975, and during the past ten years anterior fusion combined with reduction and transpedicular fixation has been performed in severe slips. Surgery was mainly indicated to alleviate pain or to prevent further progression of the slip. It seems possible to alter the natural course of the disease by operation, but it is difficult to prove the connection between the radio-graphic findings and the pain. The epidemiologic study shows that the prevalence of spondylolisthesis in Finland is 6%, but there are no significant differences between these subjects and controls without spondylolisthesis. Symptomatic patients may have pain, however, even after a long observation time. Prevention of the slip is difficult because, statistically, 90% of the slip has already occurred when the patient is first seen. Posterolateral fusion in situ is the method of choice, especially for most young patients. Anterior fusion is indicated if the slip is greater than 40%-50%. Reduction of the slip is indicated in exceptional cases only.