The process of steam raising as a source of heat or means of generating electricity using combustible fuels began with the turn of the century. From the very beginning, impurities in the fuels were responsible for added maintenance, a reduction in rate of heat transfer and corrosion due to fireside deposits of sintered or molten ash. The nature and severity of deposit formation, i.e, slagging and fouling, changed as the fuels and their impurities changed, the steam raising process evolved and the steam generators increased in size and efficiency. With the introduction of computer science, the empirical art of ash deposition from impurities in combustion gases is rapidly being transformed into the science of mineral transformation and ash deposition. This manuscript presents in chronological order an overview of the art of ash deposition while firing coal, the mechanistic approach to the problem, the recent introduction of sophisticated analytical procedures, and modeling of mineral transformations, and ash deposition underway. Adaptation of fuels such as ash oil, petroleum coke, municipal waste, wood and biomass to the steam raising process are presented individually in the order in which they were introduced. Empirical indices presently used to characterize the slagging or fouling potentials of impurities in fuels are present. Fundamental data are provided where necessary to illustrate mechanisms for ash deposition. An extensive list of key references is offered for those wishing to investigate details of any particular aspect of firesides lagging fouling or corrosion.