In less than twenty years the concept of spondylolisthesis has changed so that its occurrence is now considered fairly frequent, rather than being looked upon as a rarity. Furthermore, there is now general agreement that the term “spondylolisthesis” should be limited to those cases in which forward displacement of the vertebral body has occurred accompanied by bone defects consisting of a solution of continuity in the isthmi, or partes interarticulares, of the involved vertebrae. When such defects are present without accompanying ventral displacement of the vertebral body, the term “prespondylolisthesis, ” “spondylolysis,” “spondyloschisis” or, perhaps most logically, as suggested by Lerner and Gazin (14), “interarticular isthmus hiatus,” is used. The studies to be reported here have been limited to developing further information on the following subjects: 1. The genesis and frequency of vertebral isthmus defects with a discussion as to whether such defects are congenital or traumatic in origin. Cases of hereditary transmission are presented. 2. The frequency of occurrence of spondylolisthesis and its precursors. Some 2,080 unselected young men—candidates for the Armed Forces—were given routine lateral x-ray examinations of the lumbosacral spine to obtain this information. 3. An analysis of these cases to determine how many individuals with such defects actually have low-back pain. 4. Useful clinical signs by means of which spondylolisthesis or its precursors can frequently be detected. Etiology Just as trauma is considered a cause of bone defects in other regions of the body, so it has been indicated as responsible for spondylolysis and spondylolisthesis. A consideration of the possible influence of trauma should be divided into two parts. 1. Is the interarticular isthmus hiatus of spondylolysis itself traumatic in origin? 2. When actual forward slipping has occurred, so that by definition true spondylolisthesis is present, has the separation of the defect been occasioned by trauma, or will trauma be likely to produce symptoms in patients who already have such defects? Whether the defect in the vertebral isthmus is traumatic or congenital in origin has been the subject of considerable debate. Contrary to some of the older concepts in this country, there is now a general agreement by the proponents of the traumatic theory of etiology that such trauma must have occurred very early in life. Hitchcock (12) calls attention to the likelihood of hyperflexion during the birth process as a cause. By hyperflexion of the spine, often of moderate degree and with little force, he has been able readily to fracture the neural arch in the lower lumbar region in fetuses and stillborn children. It may seem strange, in the face of evidence to be later considered, that some conscientious investigators who have made numerous thorough examinations of fetuses and stillborn children still give credence to the traumatic theory of origin.