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A steam engine is a device that makes use of the potential energy that exists as pressure in steam, converting it to mechanical work. Early examples were the steam locomotive trains, and steam ships that relied on these engines for movement. The Industrial Revolution came about primarily because of the steam engine. The thirty seconds or so required to develop pressure made steam less favored for automobiles, which are generally powered by internal combustion engines.

The first steam device, the aeolipile, was invented by Heron of Alexandria, a Greek, in the 1st century AD, but used only as a toy. While designs had been created by varous people in the meanwhile, the first practical steam engine was patented by James Watt, a Scottish inventor, in 1769. Steam engines are of various types but most are reciprocal piston or turbine devices.

The strength of the steam engine for modern purposes is in its ability to convert raw heat into mechanical work. Unlike the internal combustion engine, the steam engine is not particular about the source of heat. Since the oxygen for combustion is unmetered, steam engines burn fuel cleanly and efficiently, with relatively little pollution.

One source of inefficiency is that the condenser causes losses by being somewhat hotter than the outside world. Thus any closed-cycle engine will always be somewhat less efficient than any open-cycle engine, because of condenser losses.

Most notably, without the use of a steam engine nuclear energy could not be harnessed for useful work, as a nuclear reactor does not directly generate either mechanical work or electrical energy - the reactor itself does nothing but sit there and get hot. It is the steam engine which converts that heat into useful work.