Earthing grids are a major part of high voltage plant at electrical installations ensuring good operation of equipment and the safety of personnel and the general public. Therefore, for improved safety, it is essential to keep the earthing grid impedance as low as possible at all times. A continuous on-line monitoring of the earthing grid is, therefore, desirable. This thesis presents the investigations and improvements to existing practice which were carried out to help the development of such a monitoring system. It also describes the specification, analysis, set up and performance tests of a prototype monitoring system. Extensive measurements in a laboratory environment, at the Cardiff University Outdoor Test Site, and at operating electrical substations and tower lines were used to validate the developed system. The literature review carried out in this work concentrated on measurement techniques used in earthing systems and safety issues related to electrical substations and tower lines. The accuracy of the existing impedance measurement system, IMS, developed at Cardiff, was examined, and the errors involved were quantified for the first time. The investigation was then concentrated on developing a fully computerised earth impedance measurement system, CEIMS, which not only allows more accurate evaluation of the test-electrode parameters but also permits savings on expensive lock-in amplifiers. The developed new algorithms allow frequency discrimination using advanced digital signal processing techniques. The extensive experimental work carried out within this programme, in the laboratory, at the Cardiff University Outdoor Test Site, and at operating substations and tower lines, has led to improvements in field test set-ups and better isolation techniques. More importantly, two new methods were developed; one is for the measurement of earth electrode resistance using the power consumption principle, while the other is for monitoring the earthing impedance of large substations. The latter technique does not require any auxiliary current injection but takes advantage of earth-wire currents flowing into/out of the substation earth grid. It is shown that if a large proportion of these currents is measured, changes to the earth grid impedance can be assessedc ontinuously.