With the exception of solar-powered systems and batteries, the electricity produced in off-grid systems is provided by a rotating generator. The most common type of generator used is the synchronous generator. The operating principles, frequency and voltage control aspects, circuit models, and efficiency of synchronous generators are described in this chapter. All generators require input mechanical power to operate. Mechanical power can come from several sources, including wind turbines, hydroturbines, and internal combustion engines. Internal combustion engines are especially common. The operating principles, efficiency, and fuel consumption of internal combustion engine-coupled generators are described in this chapter. Economic, environmental, and social considerations of the use of gen sets for off-grid electrification are discussed.