The primary goal of the Fireside Performance Indices (FPI) research project at the Energy & Environmental Research Center (EERC) was to develop a series of indices to reliably predict the fireside performance of subbituminous coals in utility boilers. Individual utilities must respond quickly and effectively to changing fuel markets because of competition within the U.S. coal-fired power industry. Spot-market purchases of coal have become commonplace. The economics associated with sulfur emissions control have caused many utilities to use Powder River Basin (PRB) subbituminous coals. The PRB coals usually provide a lower-cost, medium-heating-value, low-sulfur fuel option. Although these coals possess similar overall or bulk compositional properties, their fireside performance characteristics vary considerably within a given boiler. Consequently, bulk compositional parameters and, hence, conventional indices such as the base-to-acid ratio, stagging factor, and fouling factor are inappropriate for predicting the fireside performance of PRB coals. The development of the computer-controlled scanning electron microscopy (CCSEM) and chemical fractionation methods, however, has enabled a more thorough characterization of the inorganic constituents of PRB, coals that contribute to the following adverse operational effects: stagging, fouling, opacity, erosion and poor grindability, slag tapping, and sootblower performance. Eight predictive indices have been developed based primarily on CCSEM and chemical fractionation analysis parameters to predict the propensity of a given coal or coal blend to cause operational problems. The indices were formulated using bench-, pilot-, and full-scale combustion testing data from previous research projects combined with bench-scale data from this project to identify the primary coal inorganic properties that cause ash-related problems in utility boilers.