This book explores the opposed piston (OP) engine, a model of power and simplicity, and provides the first comprehensive description of most OP engines from 1887 to 2006. Design and performance details of the major types of OP engines in stationary, ground, marine, and aviation applications are explored and their evolution traced. The OP engine has set enviable and leading-edge standards for power/weight refinement, fuel tolerance, fuel efficiency, package space, and manufacturing simplicity. For these reasons, the OP concept still remains of interest for outstanding power and package density, simplicity, and reliability; e.g., aviation and certain military transport requirements.