A steam engine is a type of mechanical heat engine. The steam engine uses steam as a working fluid to produce mechanical work.

The history of the steam engine is a long one, going back as far as 1st Century AD. The first steam engine ever described was the Aeolipile. Described by the Hero of Alexandria, the Aeolipile consisted of a metal orb which would be filled with water. The orb was on an axis so that it could spin. Extending from the orb were curved or bent metal pipes. When the water inside was heated from a water source below, steam would come out of these tubes, causing the orb to spin on it's axis. This is not only the first recorded steam engine, but it was also the first recorded type of a steam turbine. In the next few centuries to follow, inventors created various types of steam engines for the purpose of demonstrating the properties of steam.

The first practical steam engine was developed in 1698 by English Inventor Thomas Savery. His steam engine was mainly used as a water pump. However, the Savery engine proved to have many problems, such as being prone to boiler explosions and a limited lifting height.

It wasn't until 1712 that ironmonger and Baptist lay preacher Thomas Newcomen invented the first atmospheric engine. However, the engine proved to be very inefficient, and was mainly used as a water pump.

James Watt, a Scottish inventor and mechanical engineer, developed an improved version of Newcomen's atmospheric engine. It used 75% less coal, and factories that used it no longer had to use water wheels.

The problem with both Watt and Newcomen engines was that they relied on the atmospheric pressure of steam, and not the pressure given by the expansion of steam. This meant that both required large cylinders.

In 1800, British inventor and mining engineer Richard Tevithick developed the first steam engine to use the pressure given by the expansion of steam. Not only was the engine efficient, but it also allowed for the engine to be built on a smaller scale. Thanks to Tevithick's engine, the steam engine began to be used in transportation.

Steam engines experienced a great deal of development throughout the 19th Century. Expansion (or compound) steam engines were developed, which used exhausted steam to power more cylinders. These engines provided more energy and were also more efficient. The steam engine remained the dominant source of power well into the 20th Century, with help from the highly efficient steam turbine.

Steam power saw it's decline when the internal combustion, electric and diesel engines started to become developed and put into every day use.