Geothermal waters exhibit a broad range of chemical compositions (Tables 5.1 and 5.2), from very dilute (a few hundred parts per million, by weight, of total dissolved constituents) to very concentrated (solutions containing tens of percent, by weight, of dissolved constituents). This dissolved load can provide important information about the characteristics of a reservoir including its temperature, mineralogy, and history. However, the dissolved load can also impact the performance of machinery in a geothermal power plant that is used in heating and cooling, as well as in generating power. This chapter considers the basic chemical processes that inuence the chemical properties of geothermal uids and how to use chemical analyses to gain information about the geothermal resource. Later, in Chapters 6 and 7, we will apply these principles to evaluate specic aspects of geothermal resources. In Chapter 15, we will consider the environmental aspects of the compositions of geothermal uids and how the environmental consequences of the chemical components can be mitigated.