Extensive work continues to be carried out on correlating the electric characteristics of parallel-plate discharges with an equivalent model. Theoretical models have been developed for capacitive radio frequency driven plasmas. Voltage, current and phase measurements of radio frequency discharges have been used to gain empirical data of a wide range of discharge parameters. It has been found that parasitic impedances within a plasma chamber have a substantial effect on impedance measurements of plasmas. This means that simple a priori models are inadequate for understanding plasmas. Complicated experimental setups have been used to better understand the propagation of radio frequency electromagnetic radiation through plasmas/plasma chambers. These however can not be transferred to a manufacturing environment due to non uniformity issues across a given wafer. The effect of the plasma chamber setup on plasma characteristics has been demonstrated. Although plasma processes are widely used in industry, the general understanding of these processes is poor and process control is difficult. The ability to etch fine lines and the control of anisotropy, etching rate, uniformity, selectivity and end point detection are obtained by experimental trial and error. This thesis describes a method of characterising the condition of a radio or microwave frequency excited plasma etching or deposition system as would be used in the semiconductor industry. This process can be used for: (i) monitoring the state of a system as it ages to detect when cleaning or repair is required (ii) checking that the characteristics of the system are as expected after manufacture, rebuild or modification. The location of any defects may be detected by simulating the frequency response of the altered system to see which electrical component values have changed. These can then be related to the physical components of the system. This characterisation can be integrated into the normal process flow; when the plasma is not powered up e.g., during pump down or loading for example, the network analyser can be switched in and the measurements made. In this way, the system can be characterised on a “real-time” basis.