The concentration of arsenic (As) in U.S. coal is significant in coal cleaning, coal utilization, and environmental considerations. Arsenic is significant because of its potential toxicity for plants and animals. This chapter examines concentrations and modes of occurrence of arsenic in U.S. coals. The data used in this study are from more than 5,000 determinations of As in coal samples, analyzed on an as-received basis using wet chemical and instrumental neutron activation analysis (INAA). Variation in As content was related to coal provinces and coal regions, coal rank, geologic age, sulfur and ash contents, heating values, and stratigraphic and lateral variation. The As content of foreign coal, roof- and floor-rock and coal partings, and coal wastes is discussed. The mode of occurrence of As is reviewed. It appears that the bulk of arsenic in coal is in sulfide minerals, primarily iron sulfides. Much of the arsenic-bearing sulfides may be epigenetic in origin, or the arsenic may have been emplaced by reaction of pyrite with arsenic-bearing mineralogic solutions. Low levels of arsenic (<5 ppm) may be organically associated. Analysis of the As data indicated the following. (1) Appalachian and Western Interior coals have the highest As content; Fort Union and Wind River regions have the least. (2) There is no systematic relation between As content and coal rank, coal deposit age, and total sulfur content; however, there is a sympathetic relation with pyritic sulfur content. (3) Arsenic concentration is highest in coal with heating values between 12,000 and 13,000 Btu/lb. (4) Stratigraphic profiles of As content for four areas show wide variations, with samples from western Kentucky displaying the least variation and samples from the southern Appalachians the most. Pyritic sulfur-content variations generally parallel those of As content. (5) Arsenic distribution maps of a small area in the Western Kentucky coal field generally show As content increasing from the basin toward the margin. (6) Roof and floor rocks contain similar amounts of As as in U.S. coal; arsenic content in U.S. coal is much less than that in coal wastes. (7) Arsenic enrichment factors reveal little difference on the basis of geologic age and coal rank regardless of whether average shale or the crustal averages are used for comparison. There are significant differences in enrichment factor values between some coal provinces or regions.