The basic ideas of testing and diagnosis are introduced. We start with the existence and physical nature of faults within the electronic circuit to establish the feeling on the necessity of testing and diagnosis. We explain here why under “testing” in this book we mainly understand test signal synthesis. We start with the fault and introduce the inductive fault analysis relating the faults and the possible failures. Functional and structural testing are contrasted and rationale for implementation the one and the other are given. Specialist’s terms such as testability, fault sensitization, detectability, observability, and similar are introduced. To diagnosis special attention is paid including a set of algorithms used for analog and mixed-signal circuits. We also introduce the notion on the importance of testing in conjunction with the price paid for it including the test sequence(s) design, implementing on testing hardware, and optimizing the speed of testing in mass production environment.