Live steam is steam under pressure, obtained by heating water in a boiler. The steam may be used to operate stationary or moving equipment.

A live steam machine or device is one powered by steam, but the term is usually reserved for those that are replicas, scale models, toys, or otherwise used for heritage, museum, entertainment, or recreational purposes, to distinguish them from similar devices powered by electricity, internal combustion, or some other more convenient method but designed to look as if they are steam-powered. Revenue-earning steam-powered machines such as mainline and narrow gauge steam locomotives, full-sized steamships, and the worldwide electric power-generating industry steam turbines are not normally referred to as "live steam".

Steamrollers and traction engines are popular, in 1:4 or 1:3 scale, as are model stationary steam engines, ranging from pocket-size to 1:2 scale.