Eight years ago, Dr. S. G. Hooker and M. Michel Garnier gave the 17th Louis Blériot Lecture on the same subject under the title “Power for Supersonic Flight”. Between then and now the airlines and the manufacturers have jointly evolved the new practices of health monitoring and modular maintenance, and have also made significant advances in both the basic safety of the design and the safety-monitoring of the functioning engine. More important still, the operators and manufacturers have become fully conscious of, and determined to respond constructively to, the growing public pressure to reduce pollution and technological nuisances of all kinds. Within the same eight years a complete new generation of civil engines, the large high bypass ratio turbofan, has been conceived, developed and has begun to operate in airline service. It incorporates many of the new concepts mentioned above, as well as reflecting the general progress in technology that has taken place in that time.