This chapter briefly reviews the different applications of natural circulation systems (NCSs) in various industries. Closed-loop NCSs are extensively used in solar water heaters, geothermal, fossil-fueled, and nuclear power plants. In addition, these systems are used in component-cooling applications such as transformers, electronic devices such as computer chips, and gas turbines. Although NCSs are mainly employed as heat transport devices, they can be engineered for noncooling applications such as passive controls, liquid fins, and study of low-velocity corrosion and deterministic chaos. Examples of open-loop natural circulation applications include solar chimneys, buoyancy pumps, air lift pumps, and building ventilation. The largest applications of NCSs are possibly in the chemical process industries. However, the highest power operating NCSs can be found in fossil-fueled power plants. The ESBWR (Economic Simplified Boiling Water Reactor) is the highest power (4500 MW) NCS designed so far. Mini loops with heat transport capability of the order of a few tens of Watts also can be found in the electronics industry.Some of the emerging applications of NCSs are in passive cooling of aircraft, vehicles, building ventilation, and solar thermal power plants. Due to the energy-saving potential of natural circulation, its application is increasing in various industry sectors. Natural circulation applications in electronics device cooling, solar thermal power plants, and residential buildings are expected to increase in the near future. In addition, its passive functioning without operator action or signal is important for enhanced safety and reliability of nuclear reactors, increasing their applications in the nuclear field.