Reinhardt Thiessen pioneered the thin-section method of coal petrography in the United States as well as the application of spores to the correlation of coal beds. His scientific career, mainly with the U.S. Bureau of Mines, spanned three decades during which he investigated the petrographic and botanical composition of American coals and conducted related research on peat and oil shale. He was the first in North America and, perhaps, worldwide to relate the macroscopic bands in coal to a microscopic classification of coal. Thiessen’s published record of about 82 items and the Thiessen coal thin-section slide collection, which consists of about 19,000 slides of both American and foreign coals, are a rich legacy of his monumental work in organic petrology. For his research on coal microscopy, he was awarded the Silver Medal of the Royal Society of Arts (London). The Reinhardt Thiessen Medal of the International Committee for Coal and Organic Petrology honors his name as a pioneering coal petrologist. His research in stratigraphic palynology is recognized by a tree-fern spore, Thymospora thiessenii (Kosanke) Wilson and Venkatachala.