Internet: Tremendous advances in integration technology have pushed electronic circuits to become more and more complex. On the other hand, high dependability of electronic systems has become a dominant goal in Industrial Revolution (IR4). Therefore, fault diagnosis and fault tolerance have become more important than ever before, and hence demand has risen for more awareness and research in this area. In addition, complex electronic circuits, which are embedded in safety-critical applications, need assurance of the highest level of reliability. Further, a typical manufacturing test result identifies a large number of circuit defects but even the tested circuits are subjected to failure. Added to it, exhaustive testing of circuits is an impractical approach and attaining complete fault coverage may not be feasible at any cost. Design-for-testability (DFT) and built-in self-test (BIST) techniques are used as alternate test strategies. The use of DFT and BIST methodologies requires additional hardware cost but leaving behind undetected manufacturing defects, wear and tear faults, as well as transient errors. This scenario still poses a threat to the reliable operation of the circuit. Therefore, using the concept of fault-tolerant designs is warranted to concurrently detect, diagnose, and correct a fault defect. This chapter introduces the overall concept of test methodology where fault detection and diagnosis is compactly described briefly in the context of integrated fault management. The later part of this chapter introduces the fault tolerance principles.