This paper provides a brief historical look at opposed-piston, two-stroke (OP2S) engines and their use in military applications. It also highlights the engine’s fundamental architectural advantages. In addition, the paper introduces the Achates Power opposed-piston engine, providing detailed, measured results of its power density, thermal efficiency and low heat rejection. Furthermore, the paper includes an overview of the fundamental challenges of OP2S engines, along with a discussion of how Achates Power has addressed these issues. Finally, the paper demonstrates that the operating characteristics of the Achates Power opposed-piston engine allow for optimizing the power density of the entire vehicle propulsion system as defined by Charles Raffa, Ernest Schwarz and John Tasdemir .