MILD combustion of solid fuels is a technology that burns fuels efficiently with very low emissions. Initially, it required preheating the air, but now it can burn solid fuels without preheating. Progress has been made in using low-volatile char and high-volatile biomass for MILD combustion, resulting in low NOx and particle emissions. This technology can be applied to coal, biomass, and sludge. The research defines solid-fuel MILD combustion, discusses its establishment, explores its features and emissions, and highlights the benefits of MILD oxy-fuel combustion. The future of solid-fuel MILD combustion looks promising.