Nonconvulsive status epilepticus (NCSE) is much more common than is generally appreciated. It is certainly underdiagnosed, but its presentation is protean. Diagnostic criteria and treatment are controversial. Absence status is characterized by confusion or diminished responsiveness, with occasional blinking or twitching, lasting hours to days, with generalized spike and slow wave discharges on the EEG. Complex partial status consists of prolonged or repetitive complex partial seizures (with a presumed focal onset) and produces an "epileptic twilight state" with fluctuating lack of responsiveness or confusion. There is a clear overlapping of syndromes. Other confused, stuporous, or comatose patients with rapid, rhythmic, epileptiform discharges on the EEG may have "electrographic" status and should be considered in the same diagnostic category. NCSE typically occurs following supposedly controlled convulsions or other seizures, but with persistent neurologic dysfunction despite apparently adequate treatment. Confusion in the elderly or among emergency room patients is also a typical setting. The diagnosis of NCSE usually involves an abnormal mental status with diminished responsiveness, a supportive EEG, and often a response to anticonvulsant medication. All patients have clinical neurologic deficits, but the EEG findings and response to seizure medication are variable and are more controversial criteria. The response to drugs can be delayed for up to days. Experimental models and pathologic studies showing neuronal damage from status epilepticus pertain primarily to generalized convulsive status. Most morbidity from NCSE appears due to the underlying illness rather than to the NCSE itself. Some cases of prolonged NCSE or those with concomitant systemic illness, focal lesions, or very rapid epileptiform discharges may suffer more long-lasting damage. Although clinical studies show little evidence of permanent neurologic injury, the prolonged memory dysfunction in several cases and the similarities to convulsive status suggest that NCSE should be treated expeditiously. The diagnosis is important to make because NCSE impairs the patient's health significantly, and it is often a treatable and completely reversible condition.