This chapter introduces the working principle, potential applications and current developments of free-piston engines (FPE) used for electricity generation. FPE is essentially a type of internal combustion engine (ICE) with its crankshaft mechanism removed to reduce total weightage and friction loses. However, since engine control conventionally relies heavily on crankshaft, an alternative control system is required for FPE. Therefore, studies are conducted on a free-piston linear generator (FPLG) prototype evaluating the optimum control and operation parameters to obtain best performance. © Springer Nature Singapore Pte Ltd. 2020.