Neuroleptic-induced tardive dyskinesia (TD) continues to be a serious problem in the psychopharmacology of schizophrenia. The overall mean prevalence of TD among chronically neuroleptic-treated patients is approximately 24 percent. The annual incidence in younger adults is 4 to 5 percent. Aging is a major risk factor for TD. Our ongoing prospective study suggests that the annual incidence in patients over age 45 is over 30 percent. Other likely risk factors include female gender, mood disorders, "organic" brain dysfunction or damage, diabetes mellitus, and early extrapyramidal side effects. Metoclopramide, a D2-receptor blocker commonly used in non-psychiatric medical patients, can also produce persistent TD. TD can best be assessed for research purposes by a combination of subjective and objective methods. In recent years, several instrumental procedures have been developed to objectively quantify various abnormal movements. The advantages and limitations of the traditional rating scales and the newer instrumental approaches are discussed. The course of TD is variable but often not progressive. The early theory that striatal dopamine receptor supersensitivity causes TD has now given way to the hypothesis of multiple neurotransmitter system involvement. Several animal studies have reported striatal neuronal damage with prolonged neuroleptic treatment, although its relevance to TD remains unclear. Treatments for TD, other than neuroleptic withdrawal, are still experimental.