This review article examines the impact of ash content and composition on biomass pyrolysis, focusing on four main aspects: (i) the ash content and composition in different types of biomass, (ii) the effects of ash content on catalytic pathways and biomass thermal degradation, (iii) ash-related problems during thermal degradation, and (iv) deashing techniques. Ash, which contains alkali and alkaline earth metals, heavy metals, non-metals, and other elements, significantly influences the pyrolysis process by affecting reaction rates, bio-oil yield and properties, biochar composition, and total biomass conversion. The article discusses how ash-related issues such as slagging, sintering, fouling, bed agglomeration, and corrosion can lead to operational challenges and provides an overview of deashing techniques, including biological processes, acid leaching, and additive addition. The review aims to provide new insights into the effects of ash content and composition on biomass pyrolysis, highlighting the importance of understanding and managing these factors for efficient and sustainable biomass conversion.This review article examines the impact of ash content and composition on biomass pyrolysis, focusing on four main aspects: (i) the ash content and composition in different types of biomass, (ii) the effects of ash content on catalytic pathways and biomass thermal degradation, (iii) ash-related problems during thermal degradation, and (iv) deashing techniques. Ash, which contains alkali and alkaline earth metals, heavy metals, non-metals, and other elements, significantly influences the pyrolysis process by affecting reaction rates, bio-oil yield and properties, biochar composition, and total biomass conversion. The article discusses how ash-related issues such as slagging, sintering, fouling, bed agglomeration, and corrosion can lead to operational challenges and provides an overview of deashing techniques, including biological processes, acid leaching, and additive addition. The review aims to provide new insights into the effects of ash content and composition on biomass pyrolysis, highlighting the importance of understanding and managing these factors for efficient and sustainable biomass conversion.