This compilation is offered as a source of data, references, aircraft applications, and notes for horizontally-opposed aircraft engines (HOAE) manufactured everywhere in the world. Both two-stroke cycle (2-S) and four-stroke cycle (4-S) engines are included. The author hopes that the compilation can lead to an additional understanding of the interactions during engine development by each manufacturer with their competitors on the one hand, and with the light and general-aviation aircraft designers and manufacturers using their engines on the other. Moreover, it is hoped that it can serve as a basis for anyone wishing to delve more deeply into the history and technology of these engines and their builders. The compiler will continue the search for pertinent information, engines, and applications that are presently unknown to him.