For those determined to kill in quantity and willing to kill indiscriminately, public transportation is an ideal target. Because it is public and used by millions of people daily, there is little security, with no obvious checkpoints like those at airports to inspect passengers and parcels. Passengers are strangers, promising attackers anonymity and easy escape. Concentrations of people in contained environments are vulnerable to explosives and unconventional weapons. Attacks on public transportation cause great disruption and alarm, which are the goals of terrorism. Because terrorist threats are not easily quantifiable, it is difficult to determine the right level of security. The costs of security are not determined solely by the number or capabilities of possible attackers, but also by the size and number of targets to be defended. Since 1996, the Mineta Transportation Institute (MTI) has supported a continuing research program aimed at identifying the best practices for protecting public surface transportation against terrorist attacks and other major violent crimes. This article presents a comprehensive review covering 14 transportation systems in the United States and three systems abroad. The lessons and practices of these different systems have produced some security recommendations. Effective security includes not only deterrent and preventive measures but also all efforts to mitigate casualties, damage, and disruption. Great emphasis is placed upon the mitigation of casualties through design of facilities and upon effective, rapid response that ensures safety while minimizing disruption. Crisis management planning is essential, along with constant training and field exercises involving operating entities and local authorities. Station design should include emphasis on open space, the absence of dark corners, the effective deployment of closed circuit television, installation of fire doors and blinds, ventilation shafts with reversible fans to provide rapid smoke evacuation, emergency evacuation routes, and bomb security areas. The security force should include high profile and plainclothes patrols and public involvement. Security technology should include a computerized system layout, systems to detect chemical and biological agents, and track monitoring. 2 figures, 2 appendices