This paper discusses fluctuation noise in radio receivers due to shot and thermal effects (Schottky) in the radio-frequency circuits. The r-f noise components beat with the carrier when a signal is being received and are transformed to audio components which are heard as a hissing noise. The mathematical theory for a receiver employing a square-law detector is given, and it is shown that the deflection of a meter measuring the average square of voltage or current due to the noise is proportional to the area under the curve representing the square of the over-all transmission against frequency. The overall transmission is somewhat analogous to the over-all fidelity of the receiver. This result is similar to the well-known laws for simple linear networks without frequency transformation. The method of calculating the noise due to the shot and thermal effects is discussed. Finally a convenient method of measuring the specific noise (noise per frequency interval) in a radio receiver is described, and results for several typical commercial receivers are given. The method consists in comparing the noise, as referred to the antenna circuit, with the amplitudes of the side bands, mE0, in a standard modulated signal.