A rocket is defined as an “engine or motor that develops thrust by ejecting a stream of matter rearward, or the missile or vehicle powered by such an engine.” Since the reaction principle involved assumes a self‐contained source of energy, a rocket can operate in any medium including space outside the earth's atmosphere, where there is no oxygen to support combustion. This article gives the history of the use of rockets. The laws governing these operations are discussed. The rocket is an energy conversion device that converts potential energy to thermal energy. The energy source can be chemical, electrical, beamed (solar or laser), or nuclear. Nozzle theory is described.