Lysis is common and its causes are mainly constitutional. Most of slipping has already occurred when the patient comes for consultation. A further progress is often caused by disc pathology. Neurological deficits are rare. A high-degree olisthesis and L4 location are risk factors for future back pain. Asymptomatic lysis with or without olisthesis should not be treated. A posterolateral fusion in situ without instrumentation gives good results in adolescents and young adults, whereas old patients benefit from instrumentation. Reduction cannot be recommended as a routine method.