As a result of a major technological trend towards high speed digital communications and circuits, phase noise turns out to be a relevant concern for scientists and engineers. This paper describes methods and instruments to measure the phase noise of oscillators, components and more complex devices in the radiofrequency and microwave bands, from approximately 100 kHz to 30-40 GHz, and even beyond. After a brief introduction, two sections deal with basic definitions and traditional methods, and one section presents a set of schemes that cover most actual needs. Then a new approach— known as the interferometric method— is discussed in detail, providing design strategies and examples; this method exhibits the highest sensitivity in real time, which can alse be exploited to dynamically correct the phase noise of amplifiers and oscillators. The last section deals with an improved version of the interferometric method, in which correlation is used to remove the instrument noise of two equal interferometers that simultaneously measure the same device. This scheme enables the measurement of low noise processes, even below the thermal floor, and therefore it represents the state of the art in the high sensitivity phase noise metrology.