In the process of electrical transition that vehicles are going through, also the braking system finds in regenerative braking one of the most profound innovations of last decades. The purpose of this paper is to expose this system and evaluate its effects in relation to the drivetrain. After a first description of the system, its components, its functioning and its possible benefits, some of the implications that it introduces at the transmission level will also be analysed. The considerations will be declined to the most used solutions on electric and hybrid vehicles.