Anti-NMDAR encephalitis is the most frequent autoimmune encephalitis. It predominantly occurs in children and young females. Up to 80% of patients present with severe insomnia and psychiatric and behavioural symptoms that resemble those of psychotic episodes caused by primary psychiatric diseases. In addition to the psychiatric manifestations, patients develop neurological symptoms including seizures, abnormal movements, reduced verbal output, and dysautonomic features. Up to 50% of young females have an underlying ovarian teratoma that contains nervous tissue and NMDAR, which probably trigger the immune response. Less frequently, the encephalitis is triggered by an episode of herpes simplex encephalitis probably through the release of antigens by neurons damaged by the virus. The diagnosis of anti-NMDAR encephalitis requires the demonstration of the antibodies in CSF. Up to 14% of patients do not have detectable antibodies in serum. A positive result in serum but negative in CSF must be taken with caution as these patients do not present clinical features of encephalitis and many represent false positive results. Between 80% and 90% of patients respond to treatment which includes immunotherapy and removal of the tumour when it applies.