Nystagmus is one of the most difficult subjects in neuro-ophthalmology for three reasons: first, the most common type of nystagmus is nonspecific for anatomic location and cause; second, lesions at specific sites in the posterior fossa can lead to more than one type of nystagmus; and third, specific types of nystagmus can result from lesions at various sites and have multiple possible causes. This chapter concentrates on basic principles of nystagmus, the common nonspecific types of nystagmus, and the more anatomically specific types of nystagmus. Related ocular oscillations that do not satisfy the requirements for nystagmus are discussed also.