Adult spinal deformities include a variety of three-dimensional structural alterations of the adult spine secondary to development, progression, or degeneration, and are divided into three main categories: the first is new-onset or primary degenerative scoliosis; the second is untreated idiopathic adolescent scoliosis that progresses into adulthood; and the third is secondary to vertebral structural alterations such as those resulting from surgery, trauma, or metabolic bone disease. Secondary adult scoliosis often presents as a medical imbalance caused by prior spinal surgery. The first and third categories of adult spinal deformities are the most common and clinically significant. Adult scoliosis (category I) progresses to structural curvature with age and increased disc degeneration. Accompanying the progression of disc degeneration, structural degeneration of the posterior spine leads to vertebral rotation; and persistent rotational deformity will result in ligamentous laxity and, ultimately, lateral vertebral slippage. Disruption of the intervertebral disc ligamentous complex and the ensuing degeneration of the articular synovial joints leads to abnormal motion between adjacent vertebrae, ultimately resulting in reactive changes such as endplate osteolysis, articular synovial hypertrophy/cyst formation, and hypertrophy of the ligamentum flavum. In addition, foraminal narrowing can occur on the concave side of the primary and distal segmental curves, and this narrowing is often exacerbated by disc degeneration and loss of foraminal height (superior/inferior foraminal narrowing). These changes predispose to spinal canal stenosis (central and lateral recess type) and neural foraminal stenosis, which together cause clinical symptoms in patients with adult scoliosis or spondylolisthesis. Therefore, understanding the complex pathologic mechanisms and anatomical changes involved in this degenerative process is critical for spine surgeons performing spinal deformity surgery. The prevalence of degenerative spinal deformities in adults continues to increase as the population ages and life expectancy increases. In fact, the impact of adult degenerative scoliosis on the overall public health and disease of the population has not been exaggerated, and it is likely that more patients with spinal deformities will require surgical treatment to correct the deformity while alleviating symptoms.