Impedance is a significant parameter of glow discharges that is related to electron and ion heating, wave propagation in the plasma medium, and geometrical characteristics of the vessel used for plasma processing. We present fundamental analysis of plasma conductivity and permittivity and how these parameters can be used for the definition of plasma impedance after taking into account plasma boundaries. The discussion is focused on capacitively coupled glow discharges, and the dependence of impedance on plasma microscopic parameters is reviewed. This relation is studied for different plasma regimes, and electrical equivalent circuits of glow discharges are drawn for several process conditions. Moreover, we discuss the basic aspects of maximum power transfer theory and the necessity to match the complex plasma impedance. Guidelines for the design of matching units with the support of different methodologies are given. In addition, we present the setup and instrumentation for the determination of plasma impedance from nonintrusive voltage–current (VI) measurements. The importance of characterizing and counterbalancing the reactor external circuit in order to transform the VI values from the measurement to the electrode surface plane is also highlighted. The entry ends with an example of plasma impedance measurements in capacitively coupled glow discharges of technological interest. The effect of process parameters on plasma impedance is presented and related to changes on electron density and discharge structure. The importance of such type of measurements for monitoring and optimizing specific processes is discussed together with challenges and open issues in the plasma impedance measurements and analysis field.