Even though loss of alertness has been detected in laboratory driving simulators with impressive accuracy, there are numerous scientific issues and technical challenges associated with developing a field-operational drowsiness detection and warning system. The key scientific issuesa re relatedt o the developmenot f fieldable detection models and warning systems. Issues include model validation, individualized versus generalized monitoring, and detection and warning versus activitybased maintenance. The key technical challenges are related to system operability and acceptance.C hallenges include system upkeep and calibration, driver and vehicle compatibility, risk compensation and migration, alertness restoration, and operational reliability. This paper provides an overview of the drowsy driver problem in the United States, a description of NHTSA’s drowsy driver technology program, and an introduction to some of the scientific issues and technical challenges that confront system deployment.