Unlike conventional internal combustion engines, a free-piston linear engine has no a crankshaft, and thus the pistons move freely in the cylinder. This allows a free-piston linear engine to easily adjust the compression ratio and optimize the combustion process. Free-piston linear engines include two main parts: a free-piston engine and a linear alternator. The free-piston engine is classified into three main types: single piston, dual piston, and opposed piston. The linear alternator is generally categorized as flat-type or tubular-type. Free-piston linear engines can operate with multi-fuel and HCCI combustion because of their variable compression ratios. Furthermore, they are used to generate the electric power applied in hybrid electric vehicles. To promote understanding of the unique features of free-piston linear engines, this paper presents a review of their different designs and operating characteristics. We also discuss the varied experimental systems and applications of free-piston linear engines. (C) 2016 Elsevier Ltd. All rights reserved.