It is well known that the designer bares the responsibility of the whole life cycle of the product he/she designs no matter what the product is. In that, the main concern is the functionality of the product which here is an electronic circuit or system. To ensure life-long successful performance one is to use as good technology as it is available. On the other side to ensure warning that a malfunction is lurking one is to continuously test the functionality be it in the phase of production or in phase of exploitation. Here we discuss that issue in the context of very complex circuits in which the access to the potentially faulty sites from outside becomes practically not possible. Implementation of scan-path techniques, due to the complexity, becomes also not feasible. The solution is found in the idea to redesign the circuit so that, in addition to normal function, to be capable to perform testing based on incorporated additional circuitry and to export signaturesSignature reporting the status. If these structures are activated from outside we call this concept built-in test (BIT) and if it gets activated periodically (or non-periodically, based on internal signals) it will be named built-in self-test (BIST)Built-in self-test (BIST). In this way some crucial parts of the system can be tested at low price as compared to use complex test equipment including specific test signal synthesis. To BITBuilt in Test (BIT) of analog circuits is this chapter devoted while the issue of BIT of digital circuit is out of the scope of this book since it needs complete digital system synthesis to be involved.