The commercial development of gas-charged coal reservoirs as sources of natural gas, primarily in the United States, is a recent milestone in the world hydrocarbon industry. A key factor that led to the commercial development of coalbed methane in the United States was documentation of the magnitude and extent of this gas resource. Through various studies over the last 17 years, the coalbed methane resource in 13 distinct coal basins and regions in the United States has been estimated at 9,720 to 11,720 billion m(3). Using methods developed in the US and elsewhere, the coalbed methane potential of most major coal-bearing regions in countries of the world have been estimated to be from of 89 to 269 trillion m(3) of gas in-place. However, unique characteristics of coalbed methane often preclude a direct application of previously established resource estimation methods for conventional hydrocarbons. The estimation of the volume of gas that is contained within a coal seam(s) and the estimation of the volume of gas that could be recovered from the seam(s) is dependent upon numerous physical variables. Two methods have been suggested for the estimation of the volume of gas that is contained within a coal seam: Volumetric and material balance. The more common volumetric method employs the simple, yet accurate equation that combines the volume of gas stored per unit volume of reservoir with total reservoir volume. The less commonly used material balance method employs mass-balance equations combined with reservoir pressure/porosity functions, requiring extensive knowledge and understanding of the coal seam reservoir. The assessment methodology for large basins or small prospects is similar. However, basinal or regional assessments often must employ data reduction or data simplification techniques to optimize the assessment operation and results.