Publisher Summary This chapter discusses noise in analog and digital systems. Noise from all analog perspective is a random time-varying signal that is generated in all passive and active electronic devices. It is represented as either a current or a voltage. Over a fixed time period, t, the average value of noise is zero. Analog noise, therefore, is commonly presented in terms of its mean-square value. It is sometimes also described by its root-mean-square value. Noise in digital (or large-signal) circuits is the perturbation of one nonlinear signal, the noise victim, by a second non-linear signal, the noise aggressor. The perturbation is introduced by a parasitic coupling path that is resistive, capacitive, or inductive in nature. Since digital systems are typically characterized by voltage levels, digital noise is presented in terms of voltage and voltage transients. The impact of noise differs for analog and digital systems and leads to unique equivalent circuit models and analytical techniques. Analog noise is treated by linearized expressions that correspond to small-signal equivalent circuit parameters. Digital noise is analyzed by large-signal expressions that define logic transitions. This chapter presents the characteristics of noise for both systems. Several examples are included for each type of system.