Traditional approaches to the prediction of the deposition behavior of a coal usually involve the use of empirical indices and ASTM ash fusion temperatures. These approaches, however, can give misleading results and are often unreliable. In recent years, considerable effort has been made in the development of models which overcome some of the deficiencies of the traditional approaches, as reviewed in the first part of this paper. In spite of advances, these models still fail to describe the effect of deposition on boiler operation. The second part of this paper documents the efforts in the Advanced Combustion Engineering Research Center (ACERC) to integrate an ash deposition model with a comprehensive 3D coal combustion model. An ash deposition submodel, which includes the effects of both ash chemistry and operating conditions on slagging deposits, has been incorporated into the comprehensive combustion code, PCGC-3. The submodel includes a statistically-based particle cloud model for determination of impaction rates of fly ash on boiler walls. The fraction of impacting particles that stick to the surface is determined from the physical characteristics (viscosity) of both the particles and the deposit surface. The model includes a description of deposit growth which approximates both the physical properties and chemistry of the deposit as a function of combustion conditions (operating conditions). A key feature of the model is its ability to account for the effect of deposition on operating conditions in the boiler. Simulations of deposition in both pilot-scale and utility-scale combustion facilities are reported in the paper.