Abstract: The invention is of a vehicle interlock system which utilizes non-invasive, optically based methods for detecting and measuring levels of certain target chemical substances in the blood or tissues of a user in preventing operation of the vehicle by persons exhibiting higher (or lower) than prescribed levels of such chemicals. The system of the present invention is not limited to simply measuring blood alcohol levels as are presently available breathalizer-based interlock systems, but lends itself to use in detecting unacceptable systemic levels of virtually any chemical for which the system if programmed to measure. In addition, the present system includes components for positively identifying, and during the course of vehicle operation, re-identifying the intended user and alcohol or drug user testee.