Conversion of coal into clean energy in any process either through direct combustion or conversion to gaseous and liquid fuels involves application of materials at high or reasonably high temperature in aggressive environments. Materials of construction often become the limiting factor in design of vessels and components for containment of the processes. This chapter presents a broad base of current information available on materials in the areas of (1) gaseous corrosion of metals and refractories, (2) slag corrosion of refractories, (3) erosive wear of metals and ceramics, (4) materials-design interface considerations, and (5) nondestructive inspection. It is to be noted that much of the information presented in this chapter has been obtained through significant materials research programs sponsored by the U.S. Department of Energy (Office of Fossil Energy), the Electric Power Research Institute, and the Gas Research Institute. In addition to the data obtained from research sponsored by these organizations, a great deal of information has been generated through private industries who have an interest in coal usage.