Recognizing delirium superimposed on pre-existing cognitive impairment or dementia, ‘delirium superimposed on dementia’ (DSD), is challenging because signs of delirium might be interpreted as symptoms of pre-existing cognitive dysfunction. In this paper, we review the literature on the role of electrencephalography (EEG) in the differential diagnosis of delirium, dementia and DSD. Conventional EEG, applying twenty to thirty electrodes, taking thirty minutes registration, is not feasible in psychogeriatric patients. Recent studies suggest that it is possible to reliably detect delirium using only a limited number of EEG electrodes for a short period of time. With this, use of EEG in the detection of delirium in patients with cognitive impairment or clinically manifest dementia could be possible.