Three hundred and five cases of spondylolysis with or without spondylolisthesis were reviewed. The concept of an acquired lesion must be retained even though a dysplastic lesion may be postulated to account for the familial incidence. The examination in erect position facilitates the diagnosis by favoring slipping. The vast majority of lesions are found at L5-S1. Spina bifida occulta is often found in association with spondylolisthesis. This lesion leads to apophyseal arthrosis and disc degeneration at the involved level. The accentuation of the lombosacral angle is not a predominent factor leading to spondylolysis. This angulation will increase with duration and degree of slipping. CT scan provides an excellent account of the state of the spinal canal at the affected level. Could pseudospondylolisthesis be a headed stage of spondylolysis?