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Lithium is potentially toxic to many parts of the central and peripheral nervous systems. Clinical lithium neurotoxicity may appear at any time during therapy and probably often goes unrecognised, at least for a time. Acute lithium toxicity has a mortality of 15%, and 10% of survivors suffer permanent neurological sequelae that are largely unpredictable though persons with the longest and most clinically severe intoxication are probably at highest risk. Even rapidly effective treatment with haemodialysis will not always protect against permanent residual neurological deficits. Lithium may also produce neurotoxic syndromes which develop chronically. There is a large variation among patients in relation to what constitutes a toxic serum lithium level. Both acute and chronic toxicity can occur with therapeutic range serum lithium levels. Failure to appreciate this fact may lead to delays in diagnosis and treatment, placing the patient at risk of permanent neurological damage or death. The diagnosis of lithium intoxication is largely clinical though the EEG may help if typical though non-specific EEG changes are present. If available, the red cell:plasma lithium ratio may be a sensitive indicator of intoxication. Prompt and effective treatment is indicated once the diagnosis of lithium intoxication is made. Prevention of intoxication, which requires the active involvement of both the doctor and patient, is crucial.