There are two types of spinal deformity, lordosis and kyphosis, and they are mutually exclusive at the same site. Lordosis is rotationally unstable and buckles to the side with growth and spinal flexion, producing scoliosis and changes in transverse plane geometry as secondary phenomena. Kyphosis is a uniplanar deformity arising behind the axis of spinal column rotation and it does not buckle. Spinal balance in the sagittal plane is delicate and in the normal child during adolescence both idiopathic scoliosis and idiopathic kyphosis can easily develop. The development and progression of spinal deformities can be explained in biological and mechanical terms. Any condition in which the critical load to the spine is reduced will favour the production and progression of a spinal deformity. Neuromuscular factors in idiopathic scoliosis are additive and not causative.