The inorganic components contained in coal are converted to ash during combustion and gasification. The resulting ash can cause significant problems that include slag flow behavior, ash deposition, bed agglomeration, corrosion and erosion of system parts, fine particulate that is difficult to collect, and blinding of hot-gas cleanup filters. The type of problem and the degree of the problem are dependent upon coal composition, system operating conditions, and system design criteria. Much research has been conducted on ash formation and behavior in pulverized coal- and cyclone-fired utility boilers. Many of the mechanisms of transformations and ash deposition have been worked out, and predictive models have been developed. Ash behavior research in fluidized-bed combustion systems has been conducted to determine the effects of coal quality and operating conditions on bed agglomeration and deposition on heat-transfer and other surfaces. Ash can also influence gasification system operability. Problems such as maintaining slag flow and ash deposition have been encountered. In advanced power systems, hot-gas cleanup is a significant issue in which ash plays a significant role. The issues in hot-gas cleanup include ash deposition on filters, ash removal, and vapor-phase and ash-ceramic interactions. The chemical and physical characteristics of the ash also influence their ability to be collected in air pollution control devices.