Abstract All estimates of reserves depend upon certain basic scientific principles, of which Boyle's law is fundamental, though not rigidly applicable to natural gases; a correction for deviation is required. Several methods of estimation are described herein, and illustrated with equations, diagrams, and maps. Methods are: 1. Volumetric or porosity-pressure method, based on sand thickness, pressure factors, and area. 2. Pressure-production decline method. 3. Changes in density of gas during production. 4. Associated or solution gas-cap method. 5. Comparison between fields. Condensate reservoirs and reservoirs under water drive offer particular problems. Other considerations that must be taken into account in estimating gas reserves are operation losses, compressibility of liquids, and availability of gas reserves.