The first boilers used were hay-stack-boilers. These boilers were made of copper wit a lead top. They were good enough for pressures up to 5 to 10 psi. The efficiency was low and the used materials and the shape didn't allow for high pressure.

The next generation, as introduced by Watt, were the Waggon-boilers. These boilers were much more advanced, with better appendages. Still the pressures were low, but since Watt didn't like high pressures they were sufficient for his needs.

When Travithick for the first time used high-steam pressures, what he called "strong steam", totally new boilers were needed, capable for this pressures. "High" in that time, that means something like 3 bar. His stand-alone boilers were much like the later developed Cornish boilers.

The biggest risk of a boiler is running dry. The metal of the boiler cannot resist the high temperature of the fire and therefore all parts of the boiler have to be cooled constantly by the water inside the boiler. As soon as the water level falls too low there is a risk of metal parts getting soft and weak what would cause the boiler to explode.

The Cornish or Lancashire boilers are tank-boilers with one or two big fire-tubes inside. There is a lot of water inside which makes them slow to heat up.

Another kind of boiler is the fire-tube boiler. In stead of a few big fire-tubes these boilers have a lot of smaller tubes. In this way there is a very large heated surface because of all these tubes. Disadvantage of this kind of boiler is the ease of pollution in the tubes with soot. These boilers are mostly used on steam-locomotives and traction-engines.

On ships often an adapted version of this boiler is used, the Scottish boiler (or Scotch boiler).

Modern electricity plants use water-pipe boilers. These boilers operate with very high temperatures and pressures. But since there is only little water in the tubes there is always a great risk of running dry and exploding. Therefore the boiler is very critical and it is not possible to use boilers like this without computer control.