Supercapacitors have an important role to play in the development of electric power applications, mostly in the transport domain. Acceleration and braking of cars or trains generate or require a large amount of power during a few seconds. These conditions are particularly suitable for using supercapacitors, which have a very high‐power density and a long lifetime compared to batteries. In these applications, the energetic performance depends mainly on the design of the supercapacitor modules. In automotive and rail applications, the supercapacitor modules are designed according to customer mission profiles of the vehicle. The design methods must also incorporate the constraints and the operating environment of the system (electrical stress, thermal constraints, vibration, etc.). Thus, supercapacitor modules must also work correctly and always provide the required power to the system during their lifetime. Therefore, it is vital to take into account the aging of these components in the phase of system designing. This chapter is devoted to the sizing of supercapacitor modules, taking into account the different constraints mentioned. Several designing methods are discussed and validated by simulation of some real applications. It is important to model the electrical and thermal behavior and the aging of the supercapacitor modules before the sizing. Temperature accelerates supercapacitor and module aging. A decrease in the capacitance and an increase in the series resistance are observed. Consequently, thermal management is a key issue concerning lifetime and performance of supercapacitor modules. We propose to begin this chapter by presenting the characterization, the electrical and thermal modeling and the aging of supercapacitors and finish by the sizing of supercapacitor modules.