Phase noise is one of the most important properties of oscillators that limit the capacity of high-frequency communication systems. In heterodyne conversion schemes, the phase noise of the local oscillator will be multiplied and up-converted to the transmission channel. Therefore, accurate characterization of the oscillators is highly important for the design of THz communication systems. Especially when it comes to the characterization of high-quality oscillators with extremely low phase noise, traceable measurement methods are not available. In this chapter, the mathematical model and definition of the amplitude noise (AM noise) and phase noise (PM noise) are given. Different phase noise definition standards such as single sideband (SSB) and double sideband will also be provided. Phase noise measurement techniques such as frequency discrimination and phase-locked loop (PLL) technique will be discussed. The standard two-channel cross correlation for statistical analysis of phase noise at levels below the detection limit of the phase noise receiver will be explained with mathematical formalism.