Opsoclonus-myoclonus is a pervasive neurological syndrome of children and adults. Although rare, it raises important clinical and neurobiological issues. This article provides an overview of the clinical and laboratory features, differential diagnosis, treatment, and outcome of opsoclonus-myoclonus. It pursues immunologic, genetic, electrophysiologic, neurochemical, and other clues to a pharmacologic model. Key questions include how and where the brain is injured, reversibility of the injury, possible targets for pharmacologic intervention, and which new studies are needed.