This chapter describes the principal sources of electric noise and discusses methods for the measurement of noise. The notations for voltages and currents correspond to the following conventions: dc quantities are indicated by an uppercase letter with uppercase subscripts, for example, V BE. Instantaneous small-signal ac quantities are indicated by a lowercase letter with lowercase subscripts, for example, v n. The mean-square value of a variable is denoted by a bar over the square 33-2of the variable, for example, v n 2 ¯ , where the bar indicates an arithmetic average of an ensemble of functions. The root-mean-square or rms value is the square root of the mean-square value. Phasors are indicated by an uppercase letter and lower case subscripts, for example, V n. Circuit symbols for independent sources are circular, symbols for controlled sources are diamond shaped, and symbols for noise sources are square. In the evaluation of noise equations, Boltzmann’s constant is k = 1.38 × 10−23 J K−1 and the electronic charge is q = 1.60 × 10−19 C. The standard temperature is denoted by T 0 and is taken to be T 0 = 290 K. For this value, 4kT 0 = 1.60 × 10−20 J and the thermal voltage is V T = kT 0/q = 0.025 V.