This review article, authored by Parmila Devi and Cigdem Eskicioglu, explores the effects of biochar on anaerobic digestion (AD) processes. The study highlights the challenges faced by municipal wastewater treatment plants due to the generation of organic sludge, which requires costly post-treatment through biological processes like AD. Biochar is proposed as an emerging solution to enhance AD efficiency by improving microbial activity, aiding the breakdown of complex organic compounds, increasing biogas production, and promoting reactor stability. The review focuses on the impact of biochar on various AD parameters, including methane production, lag phase, electrical conductivity, volatile fatty acids, ammonia nitrogen, pH, and oxidation-reduction potential. It also examines the inhibition of the process by biochar addition, particularly from phenols, heavy metals, and changes in microbial composition. The properties of biochar, such as feedstock type, pyrolysis temperature, specific surface area, electrical conductivity, carbon and mineral content, electron exchange capacity, aromaticity, and particle size, are discussed in detail. Key findings include that 6–16 g/L of biochar supplementation consistently yields higher cumulative specific methane compared to controls across diverse conditions and substrate types. The role of biochar is explained through four mechanisms: enhancing functional microbes, facilitating direct interspecies electron transfer (DIET), improving the degradation of refractory compounds, and increasing reactor stability. The review also addresses the uncertainties and gaps in the field, such as determining the optimal biochar dosage and type, understanding the individual and collective impact of biochar's physicochemical and redox characteristics, and formulating a clear mechanism for biochar's improvement in AD processes. The article aims to provide a comprehensive analysis of the effects of biochar on AD, drawing from extensive data on biochar properties and AD parameters.This review article, authored by Parmila Devi and Cigdem Eskicioglu, explores the effects of biochar on anaerobic digestion (AD) processes. The study highlights the challenges faced by municipal wastewater treatment plants due to the generation of organic sludge, which requires costly post-treatment through biological processes like AD. Biochar is proposed as an emerging solution to enhance AD efficiency by improving microbial activity, aiding the breakdown of complex organic compounds, increasing biogas production, and promoting reactor stability. The review focuses on the impact of biochar on various AD parameters, including methane production, lag phase, electrical conductivity, volatile fatty acids, ammonia nitrogen, pH, and oxidation-reduction potential. It also examines the inhibition of the process by biochar addition, particularly from phenols, heavy metals, and changes in microbial composition. The properties of biochar, such as feedstock type, pyrolysis temperature, specific surface area, electrical conductivity, carbon and mineral content, electron exchange capacity, aromaticity, and particle size, are discussed in detail. Key findings include that 6–16 g/L of biochar supplementation consistently yields higher cumulative specific methane compared to controls across diverse conditions and substrate types. The role of biochar is explained through four mechanisms: enhancing functional microbes, facilitating direct interspecies electron transfer (DIET), improving the degradation of refractory compounds, and increasing reactor stability. The review also addresses the uncertainties and gaps in the field, such as determining the optimal biochar dosage and type, understanding the individual and collective impact of biochar's physicochemical and redox characteristics, and formulating a clear mechanism for biochar's improvement in AD processes. The article aims to provide a comprehensive analysis of the effects of biochar on AD, drawing from extensive data on biochar properties and AD parameters.