The selection of an optimized gas adsorption model is a quantitatively challenging task when characterizing shale gas reservoirs because every model has its own assumptions and mechanisms. In addition, excess when converting different models to absolute gas adsorption is reported in the literature, which differs due to variations in adsorbed phase density or volume. The roles of controlling factors change under various conditions and consequences, and it is hard to comprehend shale storage and production sources. Quantifying the contribution of adsorbed and absorbed gases to total gas storage is also challenging despite existing advanced techniques. This paper reviews gas adsorption in the light of applications, comparisons and challenges along with shale controlling factors, and it provides a better opportunity to comprehend shale storage and platforms to investigate further mechanisms of shale gas storage. It is revealed in this review that several crucial factors are found in shale that control and affect the gas adsorption capacity. Kerogen swelling and pore shrinkage, kerogen maturity, gas type and moisture content greatly influence gas adsorption capacity. The measurement of true adsorbed and bulk amount, selection of an optimized gas adsorption model, proper investigation of shale controlling factors, and accurate measurement of adsorbed and absorbed gases may enhance the accuracy of resource assessment and production performance predictions.