The existence of electrical noise is basically due to the fact that electrical charge is not continuous but is carried in discrete amounts equal to the electron charge. Electrical noise represents a fundamental limit on the performance of electronic circuits and systems. With the explosive growth in the personal mobile communications market, the need for noise analysis/simulation techniques for nonlinear electronic circuits and systems has been re-emphasized. Even though most of the signal processing is done in the digital domain, every wireless communication device has an analog front-end which is usually the bottleneck in the design of the whole system. The requirements for low-power operation and higher levels of integration create new challenges in the design of the analog signal processing subsystems of these mobile communication devices. The effect of noise on the performance of these inherently nonlinear analog circuits is becoming more and more significant. This text presents analysis, simulation and characterization techniques and behavioural models for noise in nonlinear electronic circuits and systems, along with practical examples. The book treats the problem within the framework of, and using techniques from, the probabilistic theory of stochastic processes and stochastic differential systems. This book should be of interest to RF/analog designers as well as engineers interested in stochastic modelling and simulation.