Abstract Coal combustion for power generation is a major source of particulate air pollution. Understanding of the formation mechanisms and properties of coal combustion aerosols is critical to both the development of particulate control technologies and the assessment of their impacts on human health. This paper presents an overview of the current understanding of the mechanisms of ash particle formation in major coal-based power generation processes such as pulverized coal combustion, fluidized bed combustion and coal gasification, and a summary of the aerosol properties that are important for elucidating particle formation and evaluating their health effects. Principal particle formation mechanisms, including mineral coalescence, particle fragmentation, and vaporization-condensation, are reviewed. The complex nature of the formation mechanisms of submicron aerosols is emphasized. Evidence is provided for some solid-to-particle processes that are suspected to contribute to the formation of the submicron aerosols. Physicochemical properties such as particle size, composition and morphology are discussed. Of particular interest is the trimodality of the ash aerosol size distribution. Size-resolved elemental composition data are used to identify particle modes and to uncover their formation mechanisms. The formation of the additional central particle mode is attributed to heterogeneous condensation of vaporized species on existing fine residual ash particles. Future research topics associated with ash aerosol formation and properties are discussed.