Sorption isotherm data is of a primary importance in Gas-in-Place calculations as well as the assessment and prediction of performance of an unconventional gas reservoir. As the reservoir pressure decreases, the capacity of a CBM well to desorb methane goes down as well. This phenomenon can be very accurately measured in laboratory conditions using precise measurement and computing equipment. The data, in the form of a plot can predict the volume of gas that will be released with continued production. The storage capacity of coal to hold gas is in most cases a function of rank of the subject. This technology has been extended to perform isotherm tests on shales. While the gas storage capacities on shales can be much smaller as compared to coals, the correlation between geochemical properties and storage capacities provides good evidence that this data is as pertinent in shale reservoirs. A typical canister desorption data combined with an adsorption isotherm gives a very accurate representation of how shale gas reservoir will perform with continued production. Multiple gas isotherm tests are recommended if the percentage of certain gases other than methane is detected in gas composition analysis during the canister desorption process. Mixed gas isotherms are modeled using IAS (ideal adsorbed solution) theory of Myers and Prausnitz. Mixed gas isotherms are of major importance in ECBM (enhanced CBM) industry. This talk provides basic concepts of adsorption, how the isotherm tests are performed in a laboratory and their importance.