In the previous chapters the synthesis of test signals and algorithms for diagnosis were emphasized. To complete the subject, in this chapter the very technology of testing will be discussed. We start with the overview of failure modes of electronic components and circuits which, we expect, will familiarize the reader with the vast number of possible faults that are lurking in an electronic system. Then we will go through the life cycle of the electronic system and discuss the most frequent failures and testing methods related to that part of the life cycle. Having an overview of the problem from industrial point of view, we then address the hardware and the software needed to perform testing including the automatic test pattern generator (ATPG)Automatic test pattern generator (ATPG). The tester himself and its interface to the device under test will be studied including testing bare die (chips), encapsulated components, and printed circuit boardsPrinted circuit board. A study of the testing economy will be offered and challenges of the future will be discussed.