Reservoirs and cap rocks are the fundamental components of oil and gas reservoirs. Reservoirs serve as the storage space for oil and gas, and are the core of the reservoir system. Cap rocks are the geological layers or bodies that overlie the reservoirs and seal in the oil and gas, preventing their upward escape. They are also essential for the formation of oil and gas reservoirs. This chapter introduces the basic characteristics, types, origins, and main factors influencing the properties of reservoirs and cap rocks, with a particular focus on the characteristics, origins, and main research areas of different types of reservoirs, such as clastic rocks and carbonate rocks. The structure, characteristics, origin, and distribution of reservoirs and cap rocks are closely related to the formation of oil and gas reservoirs, the reserves of oil and gas, production capacity, and output. They directly affect the deployment and efficiency of oil and gas exploration and development.