Biochar enhances anaerobic digestion efficiency by improving microbial activity, breaking down complex organic compounds, increasing biogas production, and stabilizing reactors. This review discusses the effects of biochar on anaerobic digestion, focusing on performance, stability, biochar properties, and mechanisms. Key factors include methane production, lag phase, electrical conductivity, volatile fatty acids, ammonia nitrogen, pH, and oxidation-reduction potential. Biochar can inhibit processes through phenols, heavy metals, and microbial composition. Biochar properties are influenced by feedstock type, pyrolysis temperature, surface area, conductivity, carbon and mineral content, electron exchange capacity, aromaticity, and particle size. Biochar supplementation (6–16 g/L) consistently increased cumulative specific methane production across diverse conditions. Biochar's role is explained by four mechanisms: enhancing functional microbes, facilitating direct interspecies electron transfer, improving degradation of refractory compounds, and increasing reactor stability. Anaerobic digestion involves hydrolysis, acidification, acetogenesis, and methanogenesis. Interspecies electron transfer between fermentative bacteria and methanogens is crucial for performance. Biochar's redox active surface groups enable mediated interspecies electron transfer. Biochar properties vary based on feedstock and pyrolysis conditions. Biochar's physicochemical and redox characteristics influence digestion performance and stability. Despite studies, questions remain about optimal biochar dosage, its physicochemical and redox impacts, and the main mechanism of improvement. This review analyzes biochar dosage effects, physicochemical properties, and mechanisms. Data show biochar dosage significantly affects digestion performance. Methane production data from 194 groups show biochar dosage impacts on mesophilic and thermophilic digesters. Biochar-amended thermophilic digestion is less studied.Biochar enhances anaerobic digestion efficiency by improving microbial activity, breaking down complex organic compounds, increasing biogas production, and stabilizing reactors. This review discusses the effects of biochar on anaerobic digestion, focusing on performance, stability, biochar properties, and mechanisms. Key factors include methane production, lag phase, electrical conductivity, volatile fatty acids, ammonia nitrogen, pH, and oxidation-reduction potential. Biochar can inhibit processes through phenols, heavy metals, and microbial composition. Biochar properties are influenced by feedstock type, pyrolysis temperature, surface area, conductivity, carbon and mineral content, electron exchange capacity, aromaticity, and particle size. Biochar supplementation (6–16 g/L) consistently increased cumulative specific methane production across diverse conditions. Biochar's role is explained by four mechanisms: enhancing functional microbes, facilitating direct interspecies electron transfer, improving degradation of refractory compounds, and increasing reactor stability. Anaerobic digestion involves hydrolysis, acidification, acetogenesis, and methanogenesis. Interspecies electron transfer between fermentative bacteria and methanogens is crucial for performance. Biochar's redox active surface groups enable mediated interspecies electron transfer. Biochar properties vary based on feedstock and pyrolysis conditions. Biochar's physicochemical and redox characteristics influence digestion performance and stability. Despite studies, questions remain about optimal biochar dosage, its physicochemical and redox impacts, and the main mechanism of improvement. This review analyzes biochar dosage effects, physicochemical properties, and mechanisms. Data show biochar dosage significantly affects digestion performance. Methane production data from 194 groups show biochar dosage impacts on mesophilic and thermophilic digesters. Biochar-amended thermophilic digestion is less studied.