Alcohol impairment of driving skills appears to be due to the information processing demands of a divided attention task. Driving is characterized as a divided attention skill involving two major task components: compensatory tracking and search-and-recognition for environmental signals. Support for this position is offered from laboratory and simulator studies as well as epidemiological data.There is scant epidemiological evidence regarding marihuana effects on driving, but laboratory data are reported. Alcohol and marihuana effects differ, and it is concluded that effects of marihuana on performance are not related to division of attention and information processing rate. It is suggested that marihuana is less dangerous to the driving tasks than is alcohol, for the dose levels tested. A tentative hypothesis as to the nature of the marihuana effect is offered.