Fuels are a set of products that can serve several purposes. Alongside with oxygen, they are the combustion reagents, which, in turn, releases energy in the form of heat. In addition to its direct use, for example in stoves, ovens and boilers, heat can also be converted into other forms of energy, such as mechanical energy in the internal combustion engines of road vehicles, diesel locomotives, commercial aircraft turbines, ships and small airplanes, among others. In turn, mechanical energy, obtained from fuels, can also be converted into electricity (as is the case with small capacity generators and some power plants).

ERSE regulates the Portuguese oil sector, which includes petroleum-based fuels, namely liquid fuels (such as gasolines and diesel), and liquefied petroleum gases (propane and butane). In addition to these, ERSE regulation also covers biofuels, whose synthesis uses biological sources as raw materials.