The pathogenesis of idiopathic scoliosis is still a matter of debate as its common cause has not been found. But some basic principles of its onset and progression do exist. The curvature of the spine is always accompanied by rotation of the vertebrae and rib cage. If we want to explain the principles governing scoliosis onset and progression, we should answer some questions. First, why is side curvature of the spine always accompanied by rotation of the rib cage and vertebrae and vertebral rotation is maximal at the curve apex? Second, is structural scoliosis fixed by primary bone growth in growth plates of the vertebrae or by bone remodeling by which bones adapt to external loads? Third, why most of the curvatures are right thoracic? And fourth, what drives the progression of scoliosis? In the theory presented here, attention is given to the role of the ribs. Symmetrical rib cage fixes vertebrae in the midline of the body and prevents rotation of the vertebrae with dual articulating surfaces on each side of the vertebrae. The ribs are connected with intercostal muscles and cannot spread apart on the convex side when the spine bends. Curvature of the spine in the thoracic region is not possible without rotation of the vertebrae and deformation of the ribs. The ribs at the apex of the curve are pulled inwards towards the vertebrae because they are shifted further from the midline than other ribs. With rotation of the vertebrae at the apex of the curve, which are compressed between the ribs, the thoracic circumference diminishes and the tension in the wall is alleviated. The deformation becomes irreversible if new bone growth or resorption and remodeling change shape of the ribs and vertebrae or if ligaments are not firm enough. Bone remodeling is probably more important than primary bone growth in fixing the structural scoliosis. When the rib cage and vertebrae become structurally rotated, vertebrae lose balanced support from the ribs from both sides. Shear forces from the ribs turn vertebrae further and push vertebral bodies toward the convexity. Thus, continuous progression of scoliosis starts.