A steam engine is a heat engine that performs mechanical work using steam as its working fluid. Since the late 1700s steam engines have become a major source of mechanical power. The first applications were removing water from mines. In 1781 James Watt patented a steam engine that produced continuous rotative motion. The engines could be sited anywhere that water and coal or wood fuel could be obtained. By 1883, engines that could provide 10,000 hp had become feasible. Steam engines could also be applied to vehicles such as traction engines and the railway locomotives which are commonly just called steam engines outside America. The stationary steam engine was a key component of the Industrial Revolution, allowing factories to locate where water power was unavailable.

Today steam turbines generate about 90% of the electric power in the United States using a variety of heat sources. Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be used. The ideal thermodynamic cycle used to analyze this process is called the Rankine cycle. In the cycle, water is heated into steam in a boiler until it reaches a high pressure. When expanded through pistons or turbines, mechanical work is done. The reduced-pressure steam is then condensed and pumped back into the boiler.

In general usage, the term steam engine can refer to either the integrated steam plants (including boilers etc.) such as railway steam locomotives and portable engines, or may refer to the piston or turbine machinery alone, as in the beam engine and stationary steam engine. Specialized devices such as steam hammers and steam pile drivers are dependent on steam supplied from a separate boiler.