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There has been a recent resurgence in interest in “free piston” engine designs, particularly for small engine applications. This has been driven primarily by the demand for higher efficiency, flex-fuel capability, and also electrical demand in hybrid-electric applications. In free piston engines, expansion gasses are used to send energy directly to electrical or hydraulic power systems, rather than through a crankshaft. In many instances, the designs use opposed pistons to contribute the momentum of one piston to the upwards stroke of the opposing piston. This paper will review the details and benefits of the early free piston designs and some current designs, including our own hydraulic hybrid engine designs. The paper will then turn to a future free piston engine application that is in development.