Abstract Driving under the influence of fatigue and sleepiness is a serious safety concern. Hundreds of lives and billions of dollars are lost every year due to accidents caused by driver drowsiness. There are many aspects to the problem of a driver falling asleep while driving that include causes, detection, monitoring, warning, and countermeasures against drowsy driving. A number of crucial design issues have to be considered before the anticipated benefits of the drowsy driving warning can be fully realized. In this chapter the two major aspects, that is, warning and countermeasures, are discussed. Warning means to convey to the driver about his/her state of sleepiness/drowsiness so that corrective actions can be taken. There are many issues related to warning system design but the two main concerns are when and how to warn the driver, that is, alarm modality and alarm timing. Although there are no standard guidelines for selection and design of appropriate alarm modalities, at least three types of modalities (visual, audio, and haptic/tactile) and their combinations are possible for any alarm design. An important component of collision avoidance system is the algorithm that determines the timing of warning. A poorly timed alarm may actually undermine the safety of the driver. An alert issued too early may be ignored by drivers if they are unable to perceive the cause of the warning. On the other hand, if it occurs too late, it may be viewed as ineffective. An alarm that does not represent the true state of driver drowsiness, that is, the driver is not drowsy but the system issues a warning, is called false alarm. False and nuisance alarms are a particular problem for automotive collision avoidance and warning systems. A comprehensive review of the literature regarding driver fatigue/drowsiness warning research, the present state of research and technologies being developed, and issues related to warning/alarm design and the future trends are highlighted. Driver fatigue-related countermeasures are also discussed along with their merits and demerits.