Electrical Power Quality is the study of how close to the ideal sinusoid the voltage and current waveforms are. In reality the Voltage Quality is of most concern as it is through the terminal voltage that the devices interact. Any deviation, whether momentary or sustained, in the voltage a device sees at its terminal is a power quality (PQ) issue and could detrimentally affect the device. It is the manifestation of a disturbance in the current, that flowing through the system and its impedance, results in poor voltage quality. Devices connected to the network will be subjected to this voltage waveform and potentially be adversely affected. The important concepts are of emission, disturbance level and immunity level. This paper results from the Power Quality and GREEN Grid research programmes. It begins by reviewing the various types on power quality phenomena, their source and potential impact on generation, network of end-user loads. A critique of power quality indices is then given. Technology is ; for example the rapid uptake of photovoltaic solar power systems, EV chargers, heat pumps, and LED lights. As a consequence, the immunity of some devices is changing, which means there is a need to review and possibly change some indices. In particular voltage flicker and telephone interference indices need to be reassessed.