This thesis presents the design and application of a Plasma Impedance Monitor (PIM) as a plasma diagnostic for use with RF plasmas, focusing on its potential use in the semiconductor manufacturing industry. The plasma impedance monitor system was designed to measure the amplitude and phase of the first five Fourier components (harmonics) of the plasma voltage and current of RF plasma process reactors. The first two chapters of this thesis discuss Plasma and Plasma Process Reactors as commonly used in the manufacture of semiconductors. The most important parameters of a plasma in terms of materials processing are introduced as are the most common types of plasma process reactors and their characteristics. Chapters 3 and 4 discuss the PIM hardware and its method of calibration In chapter 3, the RF sensor and waveform sampling card, known as the IV Sensor and RF Vector Integrator respectively, are discussed in detail. The technique of measuring the RF voltage and current using the IV Sensor and the unique waveform sampling technique used by the sampling card are explained. In chapter 4, the method of calibration for the IV Sensor and RF Vector Integrator are explained. Finally, the use of a Dynamic Dummy Load (DDL) to test the performance of the system is shown. Chapter 5 demonstrates the measurement repeatability and resolution of the PIM as used with a commercial plasma process reactor. In chapter 6 some applications of the PIM for use in the semiconductor manufacturing industry are explained and demonstrated. As the manufacture of integrated circuits becomes more complex due to the reduction in component size, improved control of the manufacturing processes becomes essential. Present day plasma processing tools provide real-time information such as RF power, chamber pressure, temperature, gas flow rate and gas composition that are used to control the process. However, the reduction in device geometry, the need for higher yield, and the high cost of equipment ownership means that engineers require tighter control of the process and must be able to diagnose process and machine faults quickly and efficiently. Plasma-based processing systems can be characterised by their RF electrical parameters and it has been shown that changes in process conditions such as RF power or matching efficiency can be correlated to variability in process results and wafer quality. The use of a plasma impedance monitor to measure and control the electrical properties of the RF discharge, and ultimately detect or predict process faults, can therefore become a most valuable process diagnostic.