The National Strategy for the Physical Protection of Critical Infrastructures and Key Assets details a major part of the Bush administration's overall homeland security strategy. Implementing this strategy requires government agencies and private sector partners to identify and prioritize assets most essential to the United States' economic and social well-being. A key implementation requirement, therefore, is clear definition of what the administration considers to be critical infrastructures and key assets. While the Strategy provides the administration's definitions, along with its rationale for including specific infrastructures on the critical list, the meaning of "critical infrastructure" in the public policy context has been evolving for decades and is still open to debate. This report reviews the concept and definition of "critical infrastructure" as it has appeared in federal reports, legislation and regulation since the early 1980s. The report highlights the changes and expansion of that definition as the focus of public policy debates shifted from infrastructure adequacy to infrastructure protection. Finally the report summarizes current policy issues associated with critical infrastructure identification by federal agencies and the private sector. The report is intentionally limited to definitional issues and categorization of infrastructure. For a more general discussion of national policy regarding critical infrastructure protection, including its evolution, implementation, and continuing issues, see CRS Report RL30153, Critical Infrastructures: Background, Policy, and Implementa t ion , by [author name scrubbed]. The American Heritage Dictionary, defines the term "infrastructure" as The basic facilities, services, and installations needed for the functioning of a community or society, such as transportation and communications systems, water and power lines, and public institutions including schools, post offices, and prisons. This definition, however, and others like it, are broad and subject to interpretation. As a practical matter, what is considered to be infrastructure depends heavily upon the context in which the term is used. In U.S. public policy, the definition of "infrastructure" has been evolutionary and often ambiguous. Twenty years ago, "infrastructure" was defined primarily in debates about the adequacy of the nation's public works—which were viewed by many as deteriorating, obsolete, and of insufficient capacity. A typical report of the time, issued by the Council of State Planning Agencies, defined "infrastructure" as "a wide array of public facilities and equipment required to provide social services and support private sector economic activity." According to the report, infrastructure included roads, bridges, water and sewer systems, airports, ports, and public buildings, and might also include schools, health facilities, jails, recreation facilities, electric power production, fire safety, waste disposal, and communications services. In a 1983 report, the Congressional Budget Office (CBO) defined "infrastructure" as facilities with "the common characteristics of capital intensiveness and high public investment at all levels of government. They are, moreover, directly critical to activity in the nation's economy." The CBO included highways, public transit systems, wastewater treatment works, water resources, air traffic control, airports, and municipal water supply in this category. The CBO also noted that the concept of infrastructure could be "applied broadly to include such social facilities as schools, hospitals, and prisons, and it often includes industrial capacity, as well." In a subsequent report, however, CBO narrowed this definition of "infrastructure" to exclude some facilities often thought of as infrastructure–such as public housing, government buildings, private rail service, and schools–and some environmental facilities (such as hazardous or toxic waste sites) where the initial onus of responsibility is on private individuals. Congress, itself, has often enacted legislation defining or affecting one or more infrastructure sectors, but has rarely done so comprehensively. In 1984, Congress did enact a bill that established the National Council on Public Works Improvement with a mandate to report on the state of public works infrastructure systems ( P.L. 98-501 ). Analysis required by that act was to include "any physical asset that is capable of being used to produce services or other benefits for a number of years" and was to include but not be limited to "roadways or bridges; airports or airway facilities; mass transportation systems; wastewater treatment or related facilities; water resources projects; hospitals; resource recovery facilities; public buildings; space or communication facilities; railroads; and federally assisted housing." The Council established by P.L. 98-501 provided yet another definition of "infrastructure." The Council's report characterized "infrastructure" as facilities with high fixed costs, long economic lives, strong links to economic development, and a tradition of public sector involvement. Taken as a whole, according to the Council, the services that they provide "form the underpinnings of the nation's defense, a strong economy, and our health and safety." Under this definition of "infrastructure," the Council included highways, streets, roads, and bridges; airports and airways; public transit; intermodal transportation (the interface between modes); water supply; wastewater treatment; water resources; solid waste; and hazardous waste services. The Council's report was one of the last significant federal initiatives during the 1980s to consider the definition of "infrastructure." By the early 1990s, policy makers' attention had largely moved away from infrastructure issues broadly. Instead, legislative proposals tended to address the needs of individual infrastructure sectors. The growing threat of international terrorism in the mid-1990s renewed federal government interest in infrastructure issues. Unlike the previous period, which was focused on infrastructure adequacy, federal agencies in the 1990s were increasingly concerned about infrastructure protection. This concern, in turn, led policy makers to reconsider the definition of "infrastructure" in a security context. On July 15, 1996, President Clinton signed Executive Order 13010 establishing the President's Commission on Critical Infrastructure Protection (PCCIP). This Executive Order (E.O.) defined "infrastructure" as The framework of interdependent networks and systems comprising identifiable industries, institutions (including people and procedures), and distribution capabilities that provide a reliable flow of products and services essential to the defense and economic security of the United States, the smooth functioning of government at all levels, and society as a whole. This definition of "infrastructure" is consistent with the broad definitions from the 1980's. E.O. 13010 went further, however, by prioritizing particular infrastructure sectors, and specific assets within those sectors, on the basis of national importance. E.O.13010 stated that "certain national infrastructures are so vital that their incapacity or destruction would have a debilitating impact on the defense or economic security of the United States." The Commission's final report to the President echoed the E.O. 's definition of vital infrastructure. The general concept of "vital" or "critical" infrastructure in E.O. 13010 was not entirely new, having appeared in some form in many of the policy debates in the 1980s. The Order did break new ground, however, in listing what it considered to be critical infrastructures. According to E.O. 13010, these critical infrastructures were: telecommunications; electrical power systems; gas and oil storage and transportation; banking and finance; transportation; water supply systems; emergency services (including medical, police, fire, and rescue); and, continuity of government. The list of critical infrastructure sectors in E.O. 13010 was much broader than that reported by the National Council on Public Works Improvement. In addition to transportation, water systems, and public services—sectors with "a tradition of public sector involvement"—E.O. 13010 included infrastructures predominantly owned by private companies: telecommunications, energy, and financial services. In response to the President's Commission on Critical Infrastructure Protection final report, President Clinton signed Presidential Decision Directive 63 (PDD-63) on May 22, 1998. The Directive's goal was to establish a national capability within five years to protect "critical" infrastructure from intentional disruption. According to PDD-63, "critical" infrastructures were "those physical and cyber-based systems essential to the minimum operations of the economy and government." This definition expanded little on that in E.O. 13010, but was noteworthy for its specific mention of "cyber" infrastructure. To help achieve its goal, PDD-63 directed certain federal agencies to lead the government's security efforts and identify private sector liaisons in specific critical infrastructure sectors. These lead agencies and associated critical infrastructures are summarized in Table 1 . PDD-63 also identified certain "special functions" related to critical infrastructure protection to be chiefly performed by federal agencies: national defense, foreign affairs, intelligence, law enforcement. The first version of a National Plan for Critical Infrastructure (also called for by PDD-63) defined "critical infrastructures" as "those systems and assets—both physical and cyber—so vital to the Nation that their incapacity or destruction would have a debilitating impact on national security, national economic security, and/or national public health and safety." While the Plan concentrated on cyber-security of the federal government's critical infrastructure, the Plan refers to those infrastructures mentioned in the Directive. Following the terror attacks of September 11, 2001, President Bush signed new Executive Orders relating to critical infrastructure protection. Executive Order 13228, signed October 8, 2001, established the Office of Homeland Security and the Homeland Security Council. Among the duties assigned the Office was to coordinate efforts to protect: energy production, transmission, and distribution services and critical facilities other utilities telecommunications facilities that produce, use, store, or dispose of nuclear material public and privately owned information systems special events of national significance transportation, including railways, highways, shipping ports and waterways airports and civilian aircraft livestock, agriculture, and systems for the provision of water and food for human use and consumption. The list in E.O. 13228 is noteworthy for its specific inclusion of nuclear sites, special events, and agriculture, which were not among the sectors identified in PDD-63.