Chapter 2 introduces the foundational definitions for resilience as it applies to space systems, the concept of key system capability, resilience attributes, and evaluation criteria. The resilience attributes of avoidance, robustness, recovery, and reconstitution are introduced and defined and their impact upon system performance is illustrated. Approaches to calculating system resilience are presented, with benefits and drawbacks of each. Examples of specific types of threat mitigation techniques corresponding to the resilience attributes are also presented.