Metros and other rapid transit systems increase the mobility of urban populations while decreasing congestion and pollution. There are now 187 cities with a metro system in the world, 45 of which were inaugurated in the 21st century. The design of a rapid transit system is a hard problem involving several players, multiple objectives, sizeable costs and a high level of uncertainty. Operational research techniques cannot fully solve the problem, but they can generate alternative solutions among which the decision makers can choose, and they be employed to solve some specific subproblems. The scientific literature on rapid transit location planning has grown at a fast rate over the past 20 years. This chapter provides an account of some of the most important results. It first describes the main objectives and indices used in the assessment of rapid transit systems. It then reviews the main models and algorithms used to design such systems. The case of a single alignment and of a full network are treated separately. Then follows a section on the location of stations on an already existing network.