Since the secondary power on board has been replaced by electrical power in more electric/all electric aircraft and electrical power has become a substitution in electric propulsion, aircraft electrification is now a great innovation for aviation technology, and advanced electrical machine system is the basement for aircraft electrification. In this paper, the basic concepts and state of the art of aircraft electrification are introduced with the emphasis on its significant influence on the electrical generating system and electrical load. The opportunities and challenges which the aircraft electrical machine systems are now facing are also discussed. The advanced aircraft electrical machine systems which meet the requirements of the aircraft electrification are summarized and analyzed. The key technologies of the aircraft electrical machine systems are further listed, including new electrical material and devices, cooling methods, coupledfield analysis methods and integrated design.