The management of adult spinal deformity is characterized by significant variability in operative and nonoperative approaches. Adult spinal deformity encompasses a broad spectrum of disorders of the spine, and the disparity observed in reported clinical outcomes of operative and nonoperative care reflects the heterogeneity of the cases studied. A classification of spinal deformity in adults is important in providing a framework for comparison of similar cases and for reporting outcomes on well-defined disorders. Existing classifications of scoliosis are limited in their applicability to adult deformity because they do not include parameters of lumbar degenerative change and regional sagittal alignment that are critical to decision making in surgical care of the adult. The Scoliosis Research Society classification for adult deformity is presented in this article. The purpose of this classification is to provide a framework for reporting similar cases and to contribute to the development of an evidence-based approach to the management of adult spinal deformity.