Several textbooks and reviews have underscored the importance of atlantoaxial subluxation (AAS) in ankylosing spondylitis (AS). It has been considered as a rare feature of classical AS, yet several studies reveal that the frequency of spontaneous AAS may be as high as 21% and can cause intense pain, a myriad of neurological symptoms, and even death. In this chapter we present some anatomic considerations of the craniovertebral junction, definitions, and pertinent data on the frequency, clinical implications, and potential risk factors for AAS in primary AS patients.