After a reminder of the basic definitions of phases and constituents, as well as the different concentration units, the most common physicochemical characteristics and natural dissolved constituents are presented. Groundwater global characteristics, such as the total dissolved content, electrical conductivity, dissolved organic carbon content, electroneutrality, pH and pHs (saturation pH), Langelier saturation index, hardness, alkalinity, redox potential, saturation index, cation exchange capacity, and sodium adsorption ratio, are explained. Then, the most useful ways to represent and interpret the chemical groundwater compositions are briefly described including conventional bar and pie charts; Schoeller, Stiff, and Piper diagrams; principal component analysis; and self-organizing maps. International groundwater quality standards are summarized. The chapter ends with remarks and recommendations about groundwater sampling and monitoring strategies in order to obtain unbiased data for groundwater chemical analyses.