Blackouts from acute alcohol ingestion are defined as the inability to recall events that occurred during a drinking episode and are highly prevalent in both alcoholic and nonalcoholic populations. This article reviews the clinical manifestations, epidemiology, risk factors, cognitive impairment, and neurobiology associated with alcohol-induced blackout, with special emphasis on the neurochemical and neurophysiological basis, and gender differences. Two types of blackout have been identified: en bloc, or complete inability to recall events during a time period, and fragmentary, where memory loss is incomplete. The rapidity of rise in blood alcohol concentration is the most robust predictor of blackout. Alcohol impairs different brain functions at different rates, and cognitive and memory performance are differentially impaired by ascending versus descending blood alcohol concentration. Cognitive and memory impairment occurs before motor impairment, possibly explaining how a drinker appearing fully functional can have little subsequent memory. Blackouts are caused by breakdown in the transfer of short-term memory into long-term storage and subsequent retrieval primarily through dose-dependent disruption of hippocampal CA1 pyramidal cell activity. The exact mechanism is believed to involve potentiation of gamma-aminobutyric acid-alpha-mediated inhibition and interference with excitatory hippocampal N-methyl-d-aspartate receptor activation, resulting in decreased long-term potentiation. Another possible mechanism involves disrupted septohippocampal theta rhythm activity because of enhanced medial septal area gamma-aminobutyric acid-ergic neurotransmission. Women are more susceptible to blackouts and undergo a slower recovery from cognitive impairment than men, due in part to the effect of gender differences in pharmacokinetics and body composition on alcohol bioavailability.