Abstract: In the past 15 years, the shale gas revolution and large-scale commercial developments in the United States have driven the exploration and development of shale plays worldwide. Among many factors affecting shale gas exploration potential, the gas-bearing properties of shale (quantity, storage state, composition) and their controlling factors are the essential research attracting wide attention in the academic community. This paper reviews the research progress on the retention mechanism, influencing factors, and evaluation methods for resource potential of the shale gas system, and proposes further research directions. Sorption is the main mechanism of gas retention in organic-rich shales; the gas is mainly stored in nanopores of shale in free and sorption states. The presence of water and non-hydrocarbon gases in pores can complicate the process and mechanism of methane (CH4) sorption, and the related theoretical models still need further development. The in-situ gas content and gas-bearing properties of shale are governed by the geological properties (organic matter abundance, kerogen type, thermal maturity, mineral composition, diagenesis), the properties of fluids in pores (water, CH4, non-hydrocarbon gases), and geological conditions (temperature, pressure, preservation conditions) of the shale itself. For a particular basin or block, it is still challenging to define the main controlling factors, screen favorable exploration areas, and locate sweet spots. Compared to marine shales with extensive research and exploration data, lacustrine and marine-continental transitional shales are a further expanding area of investigation. Various methods have been developed to quantitatively characterize the in-situ gas content of shales, but all these methods have their own limitations, and more in-depth studies are needed to accurately evaluate and predict the in-situ gas content of shales, especially shales at deep depth.