This paper discusses the inter-relationship between turbojet and turbofan engines used in military and commercial service. This relationship has evolved from the modification of military engines for application to the initial commercial transport aircraft to the present where some engines have been designed from inception to meet commercial transport requirements. This trend is the result of the commercial transport engine market growing enough to justify new engine developments to meet its specific needs in some cases. There are future military and commercial propulsion requirements, however, where the anticipated market may not be sufficiently large or firm to provide the basis for a new development, for example, a medium size commercial or military STOL aircraft. Proper planning could provide the basis for development of a powerplant that would satisfy both military and commercial requirements; in fact, use of common engines may be necessary to provide a production base large enough for STOL or VTOL engine development and production to be economically viable.The J57/JT3, J75/JT4, and J52 turbojet engine; and the TF33/JT3D, and the JT8D turbofan engine programs are reviewed to show the mutual benefits that have resulted from the use of a common basic powerplant. The JT9D engine developed specifically for commercial transport use is discussed to show its relationship to military and private technology developments.