The incidence of cervical and lumbar fusion surgery has increased in the past twenty years, and during follow-up some of these patients develop changes at the adjacent segment. Recognizing that adjacent segment degeneration and disease may occur in the future does not alter the rating for a cervical or lumbar fusion at the time the patient's condition is determined to be at maximum medical improvement (MMI). The term adjacent segment degeneration refers to the presence of radiographic findings of degenerative disc disease, including disc space narrowing, instability, and so on at the motion segment above or below a cervical or lumbar fusion. Adjacent segment disease refers to the development of new clinical symptoms that correspond to these changes on imaging. The biomechanics of adjacent segment degeneration have been studied, and, although the exact mechanism is uncertain, genetics may play a role. Findings associated with adjacent segment degeneration include degeneration of the facet joints with hypertrophy and thickening of the ligamentum flavum, disc space collapse, and translation—but the clinical significance of these radiographic degenerative changes remains unclear, particularly in light of the known presence of abnormal findings in asymptomatic patients. Evaluators should not rate an individual in anticipation of the development of changes at the level above a fusion, although such a development is a recognized possibility.