Subway is an urban transportation mode designed to carry large passenger volumes in dense urban areas. Developed all around the world since the end of the 19th century, subway systems have undergone successive innovations such as deep-level construction, electrification, and automation. The transport capacity of metro systems depends on the characteristics of the network and of the rolling stock, which account for a large share of the operators’ expenses. Subway systems have been increasingly popular since the beginning of the 21st century, mainly driven by its development in Asia. Several types of pricing systems can be implemented, but in most cases transportation providers receive large subsidies. Specific topics such as economies of scale, crowding externalities, reliability, value capture and financing are also covered in this chapter.