The aim of this article is to review the paraneoplastic opsoclonus-myoclonus syndrome (POMS). Opsoclonus is characterized by involuntary, arrhythmic, chaotic, multi-directional saccades with horizontal, vertical and torsional components, and it is commonly accompanied by cerebellar ataxia and myoclonic jerks in the trunk and limbs. Parainfectious brainstem encephalitis, toxic-metabolic disturbances and others condition should be considered as potential causes of these symptoms. In adults, POMS is most commonly associated with small-cell lung cancer, breast cancer, and ovarian cancer. In children, a neuroblastoma is detected in approximately 50% of cases. Many autoantibodies have been detected in patients with POMS: this finding suggests the involvement of a humoral immune mechanism. However, most patients are seronegative for these autoantibodies. This implies that a cell-mediated immune mechanism may also be involved in the pathogenesis of opsoclonus. Although the exact pathophysiology mechanism of opsoclonus remains unclear, recent reports suggest that disinhibition of the fastigial nucleus of the cerebellum is involved. In children, the immunotherapy with corticosteroids, intravenous immunoglobulin, adrenocorticotropic hormone, plasma exchange, cyclophosphamide, or rituximab is used. Although opsoclonus is often responsive to therapy, the high incidence of sequelae related to motor function, speech, behavior, and sleep is an important problem. In adults, POMS is less responsive to immunotherapy and improves only with tumor resection. In order to develop novel and effective therapeutic strategies, further studies on the immunopathogenesis and pathophysiology of POMS are required.