Involuntary movements — or dyskinesias — are a debilitating complication of levodopa therapy for Parkinson's disease that is experienced by most patients. Despite the importance of this problem, little was known about the cause of dyskinesia until recently; however, this situation has changed significantly in the past few years. Our increased understanding of levodopa-induced dyskinesia is not only valuable for improving patient care, but also in providing us with new insights into the functional organization of the basal ganglia and motor systems.