Aviation technology progressed by leaps and bounds during the late 1930s and early 1940s. Although much of this was due to advances in airframe design, much less appreciated is the role of aero engine development. This book focuses on this aspect, particularly German piston aero engine design and development, which has been generally under researched and under published compared to Allied piston aero engines. It covers key piston aero engines such as those produced by Daimler-Benz, BMW, and Junkers, as well as less well appreciated engines such as those produced by Siemens, Argus, and Hirth. It also covers turbojets and rockets, particularly the Junkers Jumo 004 and Walter 109-509 that powered the infamous Messerschmitt Me 262 and Me 163 jet and rocket fighters. Finally, the book concludes with tables comparing Allied and German piston engines, a glossary of key terms, and a bibliography.