Large quantities of fossil fuel combustion (FFC) wastes, such as fly ash, bottom ash, flue gas desulfurization (FGD) sludge, and oil ash are being disposed of on land. There is a need to accurately assess the mobilization of elements that results from the weathering of these wastes. To help meet this need, available data on physical, chemical, mineralogical, extract, and leachate characteristics have been compiled and reviewed, and a comprehensive approach to understanding how major elements are mobilized from fossil fuel wastes is described. Many of the physical properties of fly ashes overlap those of bottom ashes; similar data for FGD sludge and oil ashes are sparse. One taxonomic and three utilitarian classification schemes have been proposed for fly ashes. However, no taxonomic system is applicable to all FFC wastes. The ranges of concentrations of Al, Ca, and Fe found in these wastes were within the ranges of their concentrations in soils. Concentrations of K, Na, Mg, and S in some FFC wastes may exceed their highest concentrations in soils. The matrix of fly ashes consists principally of quartz, glass, and mullite. As minor components in fly ashes, anhydrite, lime, periclase, hematite, and magnetite are reported frequently. A minor fraction of many of the major elements may also be present in a glassy phase. The principal compounds in FGD sludges are sulfites, sulfates, carbonates, and hydroxides of Ca. The oil ashes contain mainly sulfates and oxides of Al, Na, Fe, and Mg. Water extracts of FFC wastes contain significant fractions of Ca, Mg, K, and Na, a fact that confirms the presence of these elements in highly soluble forms. Leaching studies conducted using fly ash columns have shown that initial leachates contain high concentrations of Ca, Na, K, and S and that as leaching continues these concentrations decline and reach steady states. Usually only negligible fractions of Si, Al, Fe, and Mg are leached from fly ash columns. In this article, a comprehensive approach is suggested, one that combines thermochemical principles with chemical and mineralogical data to predict the upper limits of elemental concentrations that can be attained in leachates from weathering FFC wastes. This thermochemical approach has shown that secondary solid phases may control concentrations of Al, Ca, and S measured in extracts of fly ashes and FGD sludges and in pore waters from fly ash lysimeters at a field site. Characterization of weathered wastes and improved thermochemical data for secondary solid phases in these wastes will be useful for understanding and predicting the chemistry of leachates.