This chapter gives an in-depth look at the ways that CO2 is trapped and stored permanently in rock formations, as well as the factors that should be used to choose the best storage sites. The main trapping mechanisms consist of structural and stratigraphic trapping, which involves the physical confinement of CO2 beneath impermeable caprocks; residual trapping, where CO2 is immobilised in the pore spaces of the rock; solubility trapping, where CO2 dissolves into formation fluids; and mineral trapping, where CO2 interacts with minerals to form stable carbonates over time. While selecting geological formations for CO2 storage, the most important variables for consideration are reservoir depth, porosity, permeability, and caprock integrity to ensure effective containment. This chapter discusses the importance of site-specific risk assessments, concentrating on potential leakage pathways and the long-term stability of storage sites. Different formation types, such as depleted oil and gas reservoirs, saline aquifers, and unmineable coal seams, are assessed for their suitability based on these criteria. This chapter emphasises that effective CO2 storage is dependent on understanding the geological features of the site of storage and the mechanisms of trapping involved in it, all of which are essential to ensuring permanent confinement while minimising the adverse environmental effects.