Chapters  1 and 2 laid out the framework and guiding principles of the European electricity market. In this chapter, the basics of capacity calculation are discussed, i.e. how to determine how much (cross-zonal) transmission capacity is available for commercial exchanges. The geographical organization of the European Union into capacity calculation regions (CCRs) is discussed, and the succession of timeframes over which capacity is calculated. The capacity calculation process is described in detail: the inputs required, actors involved, and the different methodologies of capacity calculation used/applied in Europe. Then, the day-ahead capacity calculation methodology applied in Core CCR is explained through several important steps and concepts: non-costly remedial action optimization, individual validations, allocation constraints, evolved flow-based, advanced hybrid coupling. Finally, the future evolution of capacity calculation in Europe is explored.