**Abstract**

Structural failure of the spine adjacent to the level of a previous spinal fusion is commonly observed. It may be defined by the radiologic degree of adjacent deformity, often termed junctional level kyphosis, proximal junctional kyphosis or junctional level failure, or the symptomatic failure of the spine above the level of an operation, termed adjacent segment disease (ASD). ASD can be further specified according to its anatomical location of failure, which provides insight into the specific pathological cause of failure and the optimal subsequent management. This chapter describes the anatomical and pathological classification of ASD in order to help clinicians understand the cause of failure and thereby reduce its rate and offer a treatment algorithm if it occurs.

**Keywords:** adjacent segment disease, thoraco-lumbar fusion, pathological cause, junctional level kyphosis
