In this book, we concentrate on developing noise simulation techniques for RF circuits. The difference between our approach of performing noise analysis for RF circuits and the traditional techniques is that we first concentrate on the noise analysis for oscillators instead of non-oscillatory circuits. As a first step, we develop a new quantitative description of the dynamics of stable nonlinear oscillators in presence of deterministic perturbations. Unlike previous such attempts, this description is not limited to two-dimensional system of equations and does not make any assumptions about the type of nonlinearity. By considering stochastic perturbations in a stochastic differential calculus setting, we obtain a correct mathematical characterization of the noisy oscillator output. We present efficient numerical techniques both in time domain and in frequency domain for computing the phase noise of oscillators. This approach also determines the relative contribution of the device noise sources to phase noise, which is very useful for oscillator design.