This paper describes the fundamental concepts that characterize the phase modulation (PM) noise and amplitude modulation (AM) noise of electronic devices in the frequency-domain, and their relationship to tradition time-domain measures of frequency stability. The statistical confidence of the data is discussed. Using the fundamental concepts, the affects of frequency multiplication, division, and mixing on PM noise are explored. Also covered is the relationship between noise figure and PM/AM noise in an amplifier. The affect of summing a large number of similar sources or amplifiers on the resulting PM noise is briefly mentioned. Common techniques used to measure PM noise in oscillators such as single channel two-oscillator, dual channel two-oscillator, three-cornered-hat with cross corrrelation, delay line discriminator, and carrier suppression are described. It is shown how these techniques can be extended to the measurement of PM noise added by other electronic devices such as amplifiers and frequency multipliers/dividers. Common errors and the strengths and weaknesses of the various measurement and calibration techniques are also described.