This chapter is a background of necessary concepts in the field of fault tolerance and resilience. At first, we introduce the system failure lifecycle and describe the main threats for the systems, justifying the need in providing a new property. Then, the concept of resilience and its attributes and measures are reviewed. We present the vicious cycle, a view of the performancePerformance and reliabilityReliability problems that the microprocessor industry is currently facing. The classic theory of reliability is used and to demonstrate how the hardware components of an embedded system should be done to make system resilient to hardware faults. Extracting essential reliabilityReliability theory we used to analyze how reliability of a system depends on the topology and features of components. Other attributes of resilience including safety, performability, integrity, maintainability and availabilityAvailability are also reviewed. Finally, we extend the definition of resilience and apply it to the field of safety critical computing.