Mass transit is a critical infrastructure of urban environments worldwide. The public uses it extensively, with roughly 9 billion mass transit trips occurring annually in the United States alone according to the U.S. Department of Transportation data. Its benefits per traveler include lower emissions of air pollutants and energy usage and high speeds and safety records relative to many other common modes of transportation that contribute to human health and safety. However, mass transit is vulnerable to intrusions that compromise its use and the realization of the important benefits it brings. These intrusions pertain to physical conditions, security, external environmental conditions, and equity. The state of the physical condition of transit facilities overall has been summarized in the low ratings the American Society of Civil Engineers gives to mass transit, and the large dollar estimates to maintain existing conditions as well as to bring on new improvements, which are, however, many times lower than investments estimated for roadways. Security has become a growing issue, and numerous incidents point to the potential for threats to security in the US. External environmental conditions, such as unexpected inundations of water and electric power outages also make transit vulnerable. Equity issues pose constraints on the use of transit by those who cannot access it. Transit has shown a remarkable ability to rebound after crises, most notably after the September 11, 2001 attacks on the World Trade Center, due to a combination of design and operational features of the system. These experiences provide important lessons that must be captured to provide proactive approaches to managing and reducing the consequences of external factors that impinge negatively on transit.