This review article examines the role of ash content and composition in biomass pyrolysis. Ash content and composition in different types of biomass, including agricultural waste, forestry waste, municipal solid waste, and aquatic biomass, are analyzed. The effects of ash content on catalytic pathways and biomass thermal degradation are discussed, as well as ash-related problems such as slagging, sintering, fouling, bed agglomeration, and corrosion. Deashing techniques, including biological processes, acid leaching, and additives addition, are also investigated. Ash content varies among different types of biomass. Agricultural waste typically has a medium or low ash content, while forestry waste has a lower ash content. Municipal solid waste has a higher ash content, and aquatic biomass has a variable ash content. The composition of ash includes alkali and alkaline earth metals, heavy metals, non-metals, and other elements. These elements can significantly affect the thermal degradation of biomass and the properties of pyrolysis products. Ash can influence the catalytic pathway of pyrolysis, affecting the yield and properties of bio-oil, biochar, and syngas. Alkali metals such as K and Na can catalyze the thermal degradation of biomass, leading to increased biochar yield and reduced tar yield. However, they can also cause fouling and corrosion in pyrolysis reactors. Alkaline earth metals such as Ca and Mg have a minor catalytic effect on biomass thermal degradation. Heavy metals such as Fe and Pb can increase the rate of reaction and promote the formation of bio-oil, but they can also cause catalyst poisoning and deactivation. Non-metal elements such as S and Cl can deactivate catalysts and reduce bio-oil yield. Ash-related problems such as slagging, sintering, fouling, bed agglomeration, and corrosion can significantly impact the efficiency and operation of pyrolysis processes. Slagging occurs when molten ash forms and deposits on the surfaces of process equipment. Fouling occurs when non-melting ash deposits on the surfaces of process equipment. Bed agglomeration occurs when alkali silicate eutectics form and cause the aggregation of bed particles. Corrosion occurs when ash particles containing compounds such as K2CO3, K2SO4, SiO2, KCl, NaCl, and CaSO4 settle on the surfaces of process equipment. Deashing techniques such as biological processes, acid leaching, and additives addition are effective in reducing the ash content in biomass. Biological processes such as bioleaching can remove ash from biomass using microorganisms. Acid leaching can remove ash from biomass using acids such as HCl and H2SO4. Additives such as kaolin can be added to biomass to reduce ash-related problems. The selection of an appropriate deashing strategy depends on the composition and yield of biomass ash, the type of biomass, and other factors.This review article examines the role of ash content and composition in biomass pyrolysis. Ash content and composition in different types of biomass, including agricultural waste, forestry waste, municipal solid waste, and aquatic biomass, are analyzed. The effects of ash content on catalytic pathways and biomass thermal degradation are discussed, as well as ash-related problems such as slagging, sintering, fouling, bed agglomeration, and corrosion. Deashing techniques, including biological processes, acid leaching, and additives addition, are also investigated. Ash content varies among different types of biomass. Agricultural waste typically has a medium or low ash content, while forestry waste has a lower ash content. Municipal solid waste has a higher ash content, and aquatic biomass has a variable ash content. The composition of ash includes alkali and alkaline earth metals, heavy metals, non-metals, and other elements. These elements can significantly affect the thermal degradation of biomass and the properties of pyrolysis products. Ash can influence the catalytic pathway of pyrolysis, affecting the yield and properties of bio-oil, biochar, and syngas. Alkali metals such as K and Na can catalyze the thermal degradation of biomass, leading to increased biochar yield and reduced tar yield. However, they can also cause fouling and corrosion in pyrolysis reactors. Alkaline earth metals such as Ca and Mg have a minor catalytic effect on biomass thermal degradation. Heavy metals such as Fe and Pb can increase the rate of reaction and promote the formation of bio-oil, but they can also cause catalyst poisoning and deactivation. Non-metal elements such as S and Cl can deactivate catalysts and reduce bio-oil yield. Ash-related problems such as slagging, sintering, fouling, bed agglomeration, and corrosion can significantly impact the efficiency and operation of pyrolysis processes. Slagging occurs when molten ash forms and deposits on the surfaces of process equipment. Fouling occurs when non-melting ash deposits on the surfaces of process equipment. Bed agglomeration occurs when alkali silicate eutectics form and cause the aggregation of bed particles. Corrosion occurs when ash particles containing compounds such as K2CO3, K2SO4, SiO2, KCl, NaCl, and CaSO4 settle on the surfaces of process equipment. Deashing techniques such as biological processes, acid leaching, and additives addition are effective in reducing the ash content in biomass. Biological processes such as bioleaching can remove ash from biomass using microorganisms. Acid leaching can remove ash from biomass using acids such as HCl and H2SO4. Additives such as kaolin can be added to biomass to reduce ash-related problems. The selection of an appropriate deashing strategy depends on the composition and yield of biomass ash, the type of biomass, and other factors.