According to this brief, combustion boilers are widely used to generate steam for industrial applications and power generation. While all kinds of energy sources--fossil fuels, biomass, nuclear and solar energy and electricity--can be used to generate heat and steam, the scope of this brief is limited to combustion boilers using fossil fuels and biomass. The intended audience for this technology brief consists of users of techno-economic models to improve the quality of their models, energy experts, policymakers, private investors and media.