Resilience is often defined in terms of the ability to continue operations or recover a stable state after a major mishap or event. This definition focuses on the reactive nature of resilience and the ability to recover after an upset. In this chapter, we use a more general definition that includes prevention of upsets. In our conception, resilience is the ability of systems to prevent or adapt to changing conditions in order to maintain (control over) a system property. In this chapter, the property we are concerned about is safety or risk. To ensure safety, the system or organization must be resilient in terms of avoiding failures and losses, as well as responding appropriately after the fact.