The design and optimization of electronic systems often requires appraisal an of the electrical noise generated by active devices, and, at a technological level, the ability to properly design active elements in order to minimize, when possible, their noise. Examples of critical applications are, of course, receiver front-ends in RF and optoelectronic transmission systems, but also front-end stages in sensors and, in a completely different context, nonlinear circuits such as oscillators, mixers, and frequency multipliers. The rapid de velopment of silicon RF applications has recently fostered the interest toward low-noise silicon devices for the lower microwave band, such as low-noise MOS transistors; at the same time, the RF and microwave ranges are be coming increasingly important in fast optical communication systems. Thus, high-frequency noise modeling and simulation of both silicon and compound semiconductor based bipolar and field-effect transistors can be considered as an important and timely topic. This does not exclude, of course, low frequency noise, which is relevant also in the RF and microwave ranges when ever it is up-converted within a nonlinear system, either autonomous (as an oscillator) or non-autonomous (as a mixer or frequency multiplier). The aim of the present book is to provide a thorough introduction to the physics-based numerical modeling of semiconductor devices operating both in small-signal and in large-signal conditions. In the latter instance, only the non-autonomous case was considered, and thus the present treatment does not directly extend to oscillators.