The subject of visual and auditory warning devices (lights and sirens) for emergency and service vehicles is surveyed from a broad perspective. The report should provide directly useful information at all levels from the selection of hardware to a general understanding of the psychophysical factors determining the effectiveness of these devices. Topics covered include: an analysis of warning signals; the present situation and the need for uniform national standards; suggested performance standards for warning light systems and for sirens, including the reasons for the principal requirements; recommendations for actions that can be taken to improve the signal effectiveness of emergency vehicles; and brief summaries of some of the physical measurements that are made on a selection of lights and sirens.