Volcanic ash that falls into marine settings commonly alters to smectitic deposits known as bentonites, the volcanic origin of which has been recognized for many decades. However, volcanic ash falling into nonmarine coal-forming environments generally alters to kaolinitic claystones called tonsteins, and these beds have only recently been universally accepted as being volcanic in origin. The recognition of tonsteins as altered volcanic ash is based on mineralogy, texture, radiometric age, and field relations. Tonsteins occur on almost every continent, but are best known from Europe and North America. Their geologic range is coincident with that of coal-forming environments; i.e., from Devonian to Holocene. The coal-forming environment is well suited for preservation of thin air-fall deposits because it features low depositional energy, topographic depression, rapidity of burial by organic matter, and lack of detrital input due to the baffling effect of plant growth. Volcanic ashes deposited within or beneath peat beds are strongly affected by humic and fulvic acids generated from organic matter. This acidic, organic-rich, highly leaching environment is partly responsible for the alteration of volcanic glass and mineral phases into kaolinite by first-order (solution-precipitation) reactions. Bed thickness also affects ash alteration, resulting in a vertical zonation of clay mineralogy in thick beds. In addition, voluminous ash falls can have an important effect on the biological and hydrological regimes of the peat swamp. Most distal tonsteins contain a restricted suite of primary volcanic minerals, such as euhedral beta-quartz paramorphs and water-clear quartz splinters (both with glass inclusions), sanidine, idiomorphic zircon, biotite, rutile, ilmenite and magnetite, apatite, allanite, and other accessory minerals specific to a silicic magma source. Textural features indicating an volcanic air-fall origin include bimodal size distribution of components, “graupen,” accretionary lapilli, altered glass bubble junctions, and aerodynamically shaped altered glass lapilli. Radiometric dating of primary minerals in tonsteins shows that they are coeval with the stratigraphic ages of enclosing rocks. Tonstein field relations indicate an volcanic air-fall origin because they are thin, widespread, continuous layers, with sharply bounded upper and lower contacts, that often pass beyond the bounds of the swamp and are occasionally penetrated by stumps in growth position. The volcanic air-fall origin of tonsteins predicates their usefulness in many geologic studies. Because they are isochronous, tonsteins can be used to vertically zone coal beds and thus provide controls for geochemical sampling, organic petrography studies, and mine planning. Regional correlations of nonmarine strata can be made with tonsteins, and intercontinental correlations may be possible. Furthermore, the presence of clay-free volcanic-ash layers in coal beds may indicate a raised-bog origin for the peat swamp. Radiometric dating of primary volcanic minerals in tonsteins allows age determination of coal beds and the calibration of palynomorphic zones. Multiple tonsteins in thick coal beds may be useful for studying the style and history of explosive volcanism.