Lifeline is an earthquake engineering term denoting those systems necessary for human life and urban function, without which large urban regions cannot exist. Lifelines basically convey food, water, fuel, energy, information, and other materials necessary for human existence from the production areas to the consuming urban areas. Prolonged disruption of lifelines such as the water supply or electric power for a city or urbanized region would inevitably lead to major economic losses, deteriorated public health, and eventually population migration. Earthquakes are probably the most likely natural disaster that would lead to major lifeline disruption. With the advent of more and more advanced technology, the United States has increasingly become dependent on the reliable provision of lifeline related commodities, such as electric power, fuel, and water. A natural question is: What is the potential for major disruption to these lifelines, especially at the regional level? The initiation of this study by the Federal Emergency Management Agency (FEMA) is based in part on a need to better understand the impact of disruption of lifelines, from earthquakes and to assist in the identification and prioritization of hazard mitigation measures and policies. In addition, the report is intended to improve national awareness of the importance of protecting lifeline systems from earthquakes, and of assuring lifeline reliability and continued serviceability. The specific contractual requirements of this project and report are: To assess the extent and distribution of existing U.S. lifelines, and their associated seismic risk; and To identify the most critical lifelines, and develop a prioritized series of steps for reduction of lifeline seismic vulnerability, based on overall benefit. FEMA is also sponsoring a companion study to develop and demonstrate a model methodology for assessing the seismic vulnerability and impact of disruption of water transmission and distribution systems. In this initial study, lifelines of critical importance at the U.S. national level have been analyzed to estimate overall seismic vulnerability and to identify those lifelines having the greatest economic impact, given large, credible U. S. earthquakes. The lifelines examined include electric systems; water, gas, and oil pipelines; highways and bridges; airports; railroads; ports; and emergency service facilities. The vulnerability estimates and impacts developed are presented in terms of estimated direct damage losses and indirect economic losses. These losses are considered to represent a first approximation because of the assumptions and methodology utilized, because several lifelines are not included, and because, in some cases, the available lifeline inventory data lack critical capacity information.