One feature that makes coal different from conventional gas reservoirs is the manner in which the gas is stored. In conventional reservoirs, the gas exists in a free state in the pores of the reservoir rock; and its behavior can be described by the real gas law. In contrast, nearly all of the gas in coal exists in a condensed, near liquid-like state because of physical sorption. These differences bring up two important questions. How do you describe the behavior of the sorbed gas, and how do you determine the sorbed gas content? Much literature has been written concerning these two subjects, and the following discussion reviews and summarizes the results and presents the current answers to these questions. First, the behavior of the sorbed gas is described, in terms of the physical sorption process and a sorption isotherm. The changes in gas content with pressure, temperature, mineral matter, moisture, rank, petrology, different pure gases, and multicomponent sorption are discussed. Techniques for measuring isotherms are also presented. Second, methods for determining the gas content are described in terms of direct methods that actually measure the amount of gas present and indirect methods in which the gas content is inferred from the sorption isotherm. Coal, “the black rock that burns,” is the subject of song, story, and legend. The earliest literature citation of coal (combustible bodies, some of which by inference must be coal) is credited to Aristotle in his treatise “Meteorology,” which may date near the middle of the fourth century B.C. Theophrastus, a student of Aristotle, at what is probably a slightly later date provides descriptions of different forms of coal based on their behavior in combustion, identifies areas of occurrence, and states that it was used by smiths (footnote by Hoover to Agricola, 1556). Though the Greek philosophers are responsible for the earliest known literature citations, China and perhaps other parts of eastern Asia are usually believed to have preceded the Mediterranean area in recognition of coal as a peculiar material with usable properties. Inouye (1913) states that although there is no authentic record of the history of the Fu-shun coal field in southern Manchuria, “it is said that the coal was used as fuel … for copper smelting in times as remote as 2,000 or even 3,000 years ago.” Fires through most of man’s history have been fed by “traditional fuels"—wood, straw, dung, and other plant materials. That coal could be of complementary usage is recorded in the remains of funeral pyres in Wales, dated about 3,000 years ago (Lindbergh and Proverse, 1977). However, the versatility of coal was not widely appreciated, and the discovery and use of charcoal satisfied most needs of primitive metal-working. By the end of the