Low heating values, variable chemical compositions, peculiar physical properties, high investment cost and insecurity of biomass feedstocks supply limit the applications of biomass for energy and other processes. Co-firing biomass and coal has potential for the development of biomass-to-energy capacity with significant economic, environmental, and social benefits. However, co-firing is not straightforward, and some questions need to be addressed due to the differences in chemical compositions and physical properties of biomass and coal. This paper highlights key issues related to co-firing, including reactor types, feeding, hydrodynamics, ash sintering, fouling, and corrosion, based on previous studies, as well as calculations and analysis. Direct co-firing is the most common option for biomass and coal co-firing currently, mostly due to relatively low investment needed to turn existing coal power plants into co-firing plants. For direct co-firing, the physical characteristics and chemical compositions of the fuel entering the combustors or gasifiers are critical to an optimum operation. Any biomass mixed with coal needs to have acceptable physical properties. More research is needed on co-firing biomass and coal, including work on: preparation, handling, storage, and feeding of biomass feedstocks (e.g. drying, torrefaction, pelletization); co-firing mechanisms; hydrodynamic analysis of co-firing combustors and gasifiers; boiler/gasifier capacity, slagging, fouling, corrosion, efficiency, reliability, fuel flexibility; lower emissions and gas cleaning; catalyst poisoning; investment and operating costs.