Water is the "Universal Solvent." Virtually every element on the periodic table and many organic substances (molecules) are soluble (can be dissolved) to some degree in water. Many substances occur "naturally" in water—that is they are dissolved into water as it flows over rock surfaces or through aquifers in the subsurface or as it mixes with other waters. Some substances are "pollutants," having been added as the result of certain human activities, intentionally or unintentionally, including wastewater (untreated) disposal, drainage of acidic waters from abandoned mines, drainage from agricultural operations (e.g. manure, herbicides, pesticides), etc. "Water quality" implies an assessment of the degree of contamination of a water source by direct measurement of its dissolved components. Not all dissolved components in water are harmful to human health, but this depends, in part, on their concentration. In this module, we will explore some of the science and issues with respect to drinking water quality, a bit about the chemistry of natural waters, and the regulations that help ensure a satisfactory drinking water supply for the U.S. populace. In addition, we will outline some water quality issues that affect other parts of the globe.