This article is a presentation of the concept of the three-column spine. The concept evolved from a retrospective review of 41 2 thoracolumbar spine injuries and observations on spinal instability. The posterior column consists of what Holdsworth described as the posterior ligamentous complex. The middle column includes the posterior longitudinal ligament, posterior annulus fibrosus, and posterior wall of the vertebral body. The anterior column consists of the anterior vertebral body, anterior annulus fibrosus, and anterior longitudinal ligament. Major spinal injuries are classified into four different categories, all definable in terms of the degree of involvement of each of the three columns. Each type is defined also in terms of its pathomechanics, roentgenograms, and computerized axial tomograms, as well as in terms of its particular stability. The compression fracture is basically stress failure of the anterior column with an intact middle column. The burst fracture indicates failure under compression of both the anterior and middle columns. The seat-belt-type spinal fracture is the result of failure of the posterior and middle columns under tension with an intact anterior hinge. In fracture-dislocations, the structure of all three columns fails from forces acting to various degrees from one or another direction.