1) Natural or induced sleep was obtained in 97.8 per cent of the patients submitted to electroencephalography in the office, showing therefore, that sleep may be used routinely with ease. 2) In 11.6 per cent of the cases sleep records showed abnormalities in patients with entirely normal wake records. 3) In 26 per cent of the cases demonstrated abnormalities which were entirely different from those seen in the make records. 4) Most abnormalities appearing during sleep were paroxysmal in type and frequently demonstrated patterns which correlate well with different clinical types of epilepsy and offered valuables clues to prognosis and treatment. 5) During sleep, the E.E.G. was particularly useful in the study of spike-focus in the anterior temporal regions and in demonstrating discharges of thalamo-hypothalamic origin. 6) In our opinion, sleep is not to be considered an "activation technique" to be used in otherwise normal tracings, but should be done routinely in order to consider the E.E.G. complete.