This book is written based on study that has been performed with the aim of investigating the use of the partial discharge (PD)measurement technique for use as an assessment tool of the composite insulator condition. A computer-based discharge analyser was utilised to study the phase-resolved PD patterns and their statistical distributions, and also PD scatter plots. The full PD impulse waveforms with their frequency spectra were also studied by using a high frequency current transformer (HF-CT) to be able to study a wide range of frequency components of the PD signals and to study the possibility of doing on-line PD measurement in service using such a method. The results of the computer discharge analyser have shown a quite good correlation to change of the insulator surface condition. It was also found when high frequency components appeared, surface deterioration could be expected to have occurred. The use of PD measurements in identifying artificial defects on composite insulators found that PD detection of such defects also gave an indication of location of hole type defects on the insulator.