It is unclear why some children with a small magnitude scoliosis at the onset of the adolescent growth spurt develop a progressive curve. Normally the skeleton grows symmetrically, presumably because genetic and epigenetic factors regulating growth to maintain growth symmetry despite activities and environmental factors causing asymmetrical loading of the spine. This chapter reviews the recently published data relating to the notion that progression of scoliosis is a result of biomechanical factors modulating spinal growth ('vicious cycle' theory). Quantitative data exist for the key variables in an analysis of scoliosis curve progression. In a predictive model of the evolution of scoliosis simulating the 'vicious cycle' theory, and using these published data, a small lateral curvature of the spine can produce asymmetrical spinal loading that causes asymmetrical growth and a self-perpetuating progressive deformity during skeletal growth. This can occur if the neuromuscular control of muscle activation is directed at minimizing the muscular stress (force per unit cross section), although other activation strategies may produce differing spinal growth patterns. Mechanical modulation of vertebral growth is a significant contributor to the progression of an established scoliosis deformity. Quantitative simulation of this mechanism demonstrates how therapeutic interventions to alter neuromuscular control of trunk muscles or otherwise modify spinal loading may alter the natural history of progression.