Burning a fossil fuel produces heat and combustion residues. In a power generation plant boiler, for example, some of this heat is transferred from the furnace side, through the boiler tubes, into the water side. The heated water is converted into steam, which in turn drives the turbines and generators in order to produce electrical power. This process, and its inherent inefficiencies, is well understood. However, the other product of burning, namely the combustion residues, will affect this process. It is a consideration of the current status, as well as ongoing research into the monitoring and control of these residues, that form the basis for this article.