At the beginnings of the modern era of integrated electronics, due to the potential interference of the signals of the digital and analog part, separate designs were in use. In many cases that was a consequence of the differences in the frequency bands used for analog and digital circuits. For example, audio signal produced by a microphone, when converted into a digital one, will need much higher sampling frequency. The sampling-related noise, if both parts are on the same silicon die and not protective measures are undertaken, may distort the analog signal due to intermodulationIntermodulation. On the other side, the amplitudes in analog circuits usually span over several decades (sometimes even more than six) while the amplitude of the digital signal is fixed (and may be set to an arbitrarily low value exceeding the noise). In modern CMOS integrated VLSI circuits, one uses guard rings (as described in LNAE_Book 1) to prevent interference while increasing some distances also helps. So, mixed-signal integrated circuits are nowadays a reality allowing realization of complex signal processing systems on chip. As it happens, however, in this new technology, new difficulties and challenges are imposed to testing and diagnosis. This chapter is intended to slightly open the curtain and to allow the reader to become aware of the complexity of the subject of testing and to learn the main solutions.