OBJECTIVE: To review the role played by the medial longitudinal fasciculus (MLF) in ocular motor physiology and to characterize a number of syndromes that result from lesions in this eloquent brainstem tract system. BACKGROUND: The MLF is responsible for transmitting information that is crucial for the coordination and synchronization of all major classes of eye movements. A number of disease processes can produce lesions within this small yet highly strategic white matter pathway resulting in a myriad of neuro-ophthalmologic signs and symptoms. METHODS: We carefully reviewed both the literature and our collective experiences to systematically consider the neuroanatomy and physiology of the MLF and the pathophysiologic mechanisms that underlie syndromes deriving from lesions in this pathway. RESULTS: The MLF is an important structure and is composed of numerous projection systems involved in the regulation of eye movements. Pathology at this location can produce a constellation of abnormalities, many of which can be identified upon careful bedside neurologic examination. CONCLUSION: This review of the medial longitudinal fasciculus and its constituent pathways is germane to understanding a number of important principles in neuro-ophthalmology.