Nystagmus is an involuntary, rhythmic oscillation of the eye. It usually has a well-defined, oppositely oriented fast phase and slow phase component. The direction of the fast phase of nystagmus is usually defined as the direction of nystagmus. Physiological nystagmus can be induced in normal subjects under natural or experimental stimulus conditions; pathological nystagmus can occur in patients with vestibular disorders with or without external stimuli. Physiological nystagmus is induced by vestibular (temperature, rotation, or linear acceleration) or visual (optokinetic) stimuli, and it can also occur when one is straining to gaze to the side (terminal nystagmus). The characteristics of pathologic nystagmus (direction, intensity, shape) and the method of its elicitation often provide useful clues to understand the pathologic condition. Pathological nystagmus can be spontaneous (cephalometric centripetal gaze), positional (induced by changes in head position), or gaze based (caused by changes in eye position or congenital). It is influenced by fixation (light and dark, wearing a lens) and gaze position (as in congenital). The cause of pathological nystagmus can be damage to the peripheral or central vestibular system.