This paper highlights the basic concepts and advantages of More/All Electric Aircraft （MEA/AEA) in the area of aircraft industry. MEA is anticipated to optimize the aircraft performance, reduce the total cost and increase safety, reliability and robustness. The Combined Starter/Generator （CS/G） system is advised as a key technology to enable MEA/AEA. A detailed analysis for the performance requirements of CS/G system is carried out. A comparative study for different machine candidates for CS/G is proposed, while focusing on Fault-tolerance, reliability and robustness.