Adolescent idiopathic scoliosis is the single most common form of spinal deformity seen in orthopedic practice. Our knowledge about the epidemiology, etiology, natural history, and treatment has recently increased dramatically. The incidence of small curves is rather high (2% of the population), whereas severe curves are much less common (<0.1%), but we cannot always predict which curve will progress. Abnormalities of the neuromuscular system and of calcium metabolism, and certain growth, genetic, and mechanical factors may all play roles in the pathogenesis of the disorder. The physiologic secondary effects of severe scoliosis relate to restrictive lung disease, but most patients do not have a deformity great enough to affect their cardiorespiratory function. The psychological and social effects of scoliosis are significant for patients but difficult to quantitate. For most patients with moderate scoliosis-that is, more than 25 to 30 degrees-treatment with an underarm brace or electrical stimulation is adequate to "control" progression of the curve. Surgical fusion allows actual correction of the curve but is indicated in only a small percentage of patients-usually those with more than 50 degrees of deformity.