The discipline of geochemistry provides insights into how the natural environment impacts animal and human health and is the basis for the important subdiscipline of medical geochemistry. Among the more important contributions of medical geochemistry are the maps illustrating the distribution, on various scales, of potentially toxic trace elements. Chemical analyses of surface water and groundwater, stream sediments, and soil horizons have been published by numerous countries covering large geographic regions. Among the most comprehensive compilations is the Geochemical Atlas of Europe containing analytical data on more than 50 elements from stream water, stream sediment, and three soil horizons in 26 countries. Geochemical processes play a variety of important roles in controlling how humans are exposed to potential toxicants in a wide range of geogenic or anthropogenic materials. Once taken up by the body, geogenic materials such as dusts, soils, and water and their contained toxicants can react chemically with the body's fluids, and these chemical interactions can play key roles in toxicity. In addition to the harmful effects of some geogenic materials, certain clays have demonstrated remarkable antimicrobial properties when applied to open wounds with bacterial infections. Numerous case studies illustrate the potential human health impacts of organic compounds from geogenic sources, and especially those from fossil energy deposits. This is a challenging area of study since disease(s) resulting from exposures may be chronic rather than acute, and involve complex mixtures of substances. Medical geochemistry can play a key role in helping to protect the safety of drinking water by identifying the sources, concentrations, and forms of potentially harmful elements such as arsenic, mercury, and fluorine in natural waters. Chemical and mineralogical characterization of coals has helped to identify the sources of health problems afflicting millions of people worldwide.