second part concentrates on survey techniques explaining how these should be performed and the procedures which need to be adopted to ensure reliable sampling and analytical data are obtained. A geothermal system requires a heat source and a fluid which transfers the heat towards the surface. The fluid could be molten rock (magma) or water. This book concentrates on the chemistry of the water, or hydrothermal, systems. Consequently, magma-energy systems are not considered. Hot-dry rock (HDR) systems are similarly outside the scope of this text, principally because they contain no indigenous fluid for study. Both magma-energy and HDR systems have potential as energy sources but await technological developments before they can be exploited commercially. Geothermal systems based on water, however, are proven energy resources which have been successfully developed throughout the world.