Factors affecting ash deposition in pulverized fuel (pf)-fired furnaces include the nature of the coal mineral matter, the metallurgy of the furnace tubing, and operational considerations. The minerals in the coal exhibit responses that range from decomposition or vitrification through structural breakdown to retention of their original structure because they are stable at the temperatures involved. Furnace deposits consist of minerals and glasses derived from the coal mineral matter. Elements can move out of decomposed minerals into glasses if the source materials allow for higher diffusion rates. Upon cooling, crystallization occurs appropriate to the glass composition. This paper reviews the techniques used for mineralogical evaluation, the mechanisms of deposit formation, and interactions of the coal minerals that may cause deposits to develop. Many of the currently accepted mechanisms for deposit attachment and accumulation appear to be without any mineralogical/chemical foundation and require reconsideration in light of the reactivity of the different mineral matter components.