The design principles of the splined-disc-type rotor with Hirth-serration couplings used in all Siemens heavy-duty gas turbines since 1960 are explained. The benefits of this rotor construction are elaborated against the operational requirements imposed on rotors by present-day large advanced gas turbines for electric power generation. Details of the stress assessment, analysis and testing underlying the design, as well as a description of the assembly of the rotor are also presented, using mostly the 170 MW-class 3600 rpm Model V84.3A gas turbine to illustrate the most recent technology.