Carbon capture and storage (CCS) is one of the methods for the reduction of anthropogenic CO2 emissions. Deep saline aquifers, and depleted oil and gas fields are all potential storage sites but they differ substantially for the quantity of CO2 they can accommodate, and for the knowledge level and uncertainties characterizing them. Typically, depleted fields have a relatively limited capacity but they have been thoroughly investigated and information about their geological structure and rock properties are already available before conversion into a storage facility. Deep aquifers are potentially able to host very large CO2 quantities but they are mostly unexplored traps and require significant investments to assess their suitability as safe CO2 storage sites.