The implementation of CO2 capture and geological storage (CCGS) technology to significantly reduce CO2 emissions necessitates a thorough understanding of available CO2 storage capacity. This capacity can be assessed at various scales, from country to site-specific levels, each offering different levels of detail and resolution. For depleted oil and gas reservoirs, CO2 storage capacity is straightforward to estimate based on recoverable reserves, reservoir properties, and in situ CO2 characteristics. In CO2-EOR, this capacity can be roughly evaluated using worldwide field experience or more accurately through numerical simulations. Coal beds' theoretical CO2 storage capacity is determined by coal thickness, CO2 adsorption isotherms, and recovery and completion factors. Deep saline aquifers, however, present a complex challenge due to multiple trapping mechanisms operating simultaneously, requiring detailed data for reliable and accurate estimation, which is only feasible at the local and site-specific scales. Despite defined methodologies, significant challenges remain, including data scarcity, particularly for coal beds and deep saline aquifers, and a lack of knowledge about the coefficients that reduce theoretical to effective and practical storage capacity, as well as the interplay between various trapping mechanisms in deep saline aquifers.The implementation of CO2 capture and geological storage (CCGS) technology to significantly reduce CO2 emissions necessitates a thorough understanding of available CO2 storage capacity. This capacity can be assessed at various scales, from country to site-specific levels, each offering different levels of detail and resolution. For depleted oil and gas reservoirs, CO2 storage capacity is straightforward to estimate based on recoverable reserves, reservoir properties, and in situ CO2 characteristics. In CO2-EOR, this capacity can be roughly evaluated using worldwide field experience or more accurately through numerical simulations. Coal beds' theoretical CO2 storage capacity is determined by coal thickness, CO2 adsorption isotherms, and recovery and completion factors. Deep saline aquifers, however, present a complex challenge due to multiple trapping mechanisms operating simultaneously, requiring detailed data for reliable and accurate estimation, which is only feasible at the local and site-specific scales. Despite defined methodologies, significant challenges remain, including data scarcity, particularly for coal beds and deep saline aquifers, and a lack of knowledge about the coefficients that reduce theoretical to effective and practical storage capacity, as well as the interplay between various trapping mechanisms in deep saline aquifers.