The September 11 attacks triggered concern about the performance of “critical” infrastructure on which social, political, and economic activity depend. The attacks moved terrorism to the top of the national security agenda and led to significant legislative, regulatory, and behavioral changes. Furthermore, the shift in focus to the threat of terrorism diminished policymakers’ appreciation and preparation for the natural disasters that communities typically face every year (Boin and McConnell 2007). The increasing number of declared natural disasters, coupled with the threat of terrorism, suggests that “extreme events” can lead to failures in critical infrastructure. These failures have national implications that can ripple through society and the economy. This chapter is about the performance of our interdependent infrastructure systems in extreme events, which are outside shocks to infrastructures; we do not consider failures internal to a system, such as major power blackouts that are not triggered by some significant external shock. We argue that “infrastructure” is best considered as systems of technical and social systems that interact in both predictable and unpredictable ways. As such, we cannot simply consider their design and performance as solely technological problems. There is no one universally accepted definition of “infrastructure.” The Compact Oxford English Dictionary defines the term as “the basic physical and organizational structures and facilities (e.g., buildings, roads, power supplies) needed for the operation of a society or enterprise” but uses the example sentence “the social and economic infrastructure of a country,” suggesting that the term is very broad and very vague. The term came into widespread use in the 1960s and 1970s to mean “public works” (Boin and McConnell 2007). Alternative definitions link “public works” with narrowly defined systems, such as telecommunications and electrical systems, as well as broader systems such as finance, health care, and food production and distribution. The broader definition of infrastructure, which gained currency after September 11, refers to what’s become known as “critical” infrastructure.