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A stable frequency source is a fundamental requirement in virtually every RF and wireless system, yet all real-world oscillators exhibit some degree of short-term frequency instability known as phase noise. This instability manifests as unwanted sidebands around the carrier in the frequency domain and as timing jitter in the time domain. When phase noise is excessive, it causes spectral regrowth that leaks energy into adjacent channels, reduces receiver sensitivity through reciprocal mixing, and rotates digital modulation constellations to the point where bit errors multiply. Understanding these effects is essential for engineers designing transmitters, receivers, and frequency synthesizers for modern communications standards. This guide walks through the physics of phase noise, its practical consequences for system performance, and the two principal measurement approaches — the traditional spectrum analyzer method and the more sensitive cross-correlation technique used in dedicated phase noise analyzers — giving engineers the knowledge they need to specify, measure, and minimize phase noise in their designs.