This paper describes mobile power quality monitoring systems developed to carry out extensive power quality (PQ) measurements nationwide, in addition to instrumentation and measurement technologies used in those systems. The developed systems have been used to measure the PQ of the Turkish Electricity Transmission System, which is among the largest transmission systems in Europe. To make trouble-shooting studies at the problematic measurement points in terms of PQ different measuring instruments and a wide range of PQ analysis methods from raw data acquisition to PQ event detection have been achieved depending on the nature of the problem, thanks to the flexible signal processing algorithms developed. Being an essential issue in the evaluation of PQ at various points of the electricity transmission network, time synchronization of mobile PQ monitoring systems has been applied using different techniques. Electromagnetic immunity of the system has been tested and reported. Based on the proposed instrumentation and measurement technologies, PQ of the 205 measurement points has been assessed all over the country. In order to exhibit the capabilities and usefulness of the mobile monitoring system developed, sample measurement results are given in the paper. Guidelines of instrumentation and measurement technologies are presented for other researchers who would like to build distributed PQ measurement systems. (C) 2008 Elsevier Ltd. All rights reserved.