The issues of combustion technology are today more than ever dominated by questions regarding ash and slag behavior. Therefore, there has been a desire among power plant engineers to gather and deepen knowledge about the behavior of the mineral substances in fuels. This work attempts to provide answers to the numerous questions: it has evolved from a presentation given to the combustion working group in the committee for operational experiences of the Association of Large Boiler Owners, in which the current state of knowledge and technology was briefly outlined, and the necessity for further in-depth work was emphasized as an urgent collective task. There is a call for a revision of our views and our research methods due to the rapid advancements in combustion and boiler technology.