As the first chapter of a book on digital system test, this chapter tries to cover some of the basics. Also, as the first chapter of a book that emphasizes the use of HDLs in test, we try to cover HDL-based design flow and discuss how various test applications fit in this process, and we show various places where HDLs can help the test process of digital systems. In this chapter we try to answer some of the important questions about digital system testing. The primary questions that need to be answered are what it is that we are testing in digital system test, and why we are testing it. The answer to these questions are: A manufactured chip or device, which will be elaborated in the sections that follows, and we are testing the device for physical defects. Other questions that remain to be answered are regarding the methods that we use for testing, ways of making a chip or a manufactured device more testable, how HDLs can help the test process, and finally what constraints we have in digital system testing.