Conductivity has been the simplest, oldest, most common, most reliable and lowest cost measurement used for determining water purity. Nevertheless, this lowly measurement has provided the means for protecting and assuring efficient operation of billions of dollars worth of power and steam generation equipment throughout the world for over three-quarters of a century. Recent innovations in measurement accuracy, temperature compensation, and sample conditioning have enabled this basic detection method to provide more information and to expand its use even further. From the simplest TDS meter to the most sophisticated ion chromatograph, conductivity measurement is the core technology. The widespread use of this measurement often outpaces the understanding of it. Provided here are the fundamentals and application of conductivity measurement as used in power and steam generation, including ionic conductance, cell constant, measuring techniques and temperature effects. In addition, various means of sample conditioning are discussed, covering the measurements of cation conductivity, degassed cation conductivity, pH and CO2 calculations and even TOC (total organic carbon) measurements. It is hoped this discussion will assist those applying conductivity measurement in any of its incarnations to have a greater understanding and to obtain better results.