Biomass combustion has a huge potential to produce power and heat in a sustainable way. However, some biomass fuels have high potassium contents, which react with other ash forming elements (i.e. Cl, Si, P and S) and lead to different ash related operational problems. Utilization of additives to abate these problems have been studied and tested for several decades. This work reviews current knowledge and studies about properties and effects of additives reported in the literature. Various additives can mitigate ash related issues by the following possible mechanisms: 1) capturing problematic ash species via chemical adsorption and reactions, 2) physical adsorption and elutriating troublesome ash species from combustion facilities, 3) increasing the biomass ash melting temperature by enhancing inert elements/compounds in ash residues, and 4) restraining biomass ash sintering by diluting and powdering effects from the additives. Additives are grouped according to the contained reactive compounds, including Al-silicates based additives, sulphur based additives, calcium based additives, and phosphorous based additives. Additives with strong chemical adsorption and reaction capacities can minimize K related ash sintering, deposition and slagging during biomass combustion processes. The effective chemical reaction mechanisms are closely related to K-Al-Si, K-Ca-Si and K-Ca-P systems. The capacities of additives to reduce ash related problems are heavily influenced by mass/molar ratios between the reactive components in the additives and the problematic elements in the biomass ash, as well as the reaction atmosphere and combustion technology. More detailed studies on high temperature reactions between additives and ashes from biomass combustion are needed