The term encephalopathy encompasses a wide variety of complex syndromes caused by a large number of different toxic, metabolic, infectious, and degenerative derangements. Acute encephalopathy typically presents with a fluctuating course involving alteration of mental status or confusion and decreased (or rarely increased) motor activity. There usually are lethargy, cognitive impairment, altered memory and mental processing of information, and disturbed sleep–wake cycles. Encephalopathy mainly occurs in the elderly and is frequently encountered in intensive care units and postoperatively. Despite new diagnostic procedures and advances in intensive medical care, acute encephalopathy constitutes a significant cause of morbidity and mortality in hospitalized patients. EEG enables rapid bedside electrophysiological monitoring providing dynamic real-time information on neocortical brain activity and dysfunction. Hence, EEG complements clinical and neuroimaging assessments of encephalopathic patients. Progressive slowing of EEG background activity with increasing cerebral compromise, the emergence of episodic electrographic transients, seizures, and decreased EEG reactivity to external stimuli provide important diagnostic and prognostic information. The aim of this review was to provide a comprehensive overview of the current evidence for the diagnostic and prognostic value of EEG in adult intensive care unit patients with acute nonhypoxic encephalopathy.