This chapter examines the impact of biochar amendments on the composting process, with a focus on microbial dynamics in biochar-compost mixtures. Biochar, recognized for its large surface area and high porosity, is proposed as a compost additive due to its ability to enhance microbial diversity, abundance, and activity. These enhancements are achieved by altering the physicochemical characteristics of decomposing organic residuals, thus stimulating the composting process and improving compost quality. Our analysis reveals that the intrinsic properties of the organic material used for biochar production, along with the pyrolysis temperature, significantly influence microbial diversity and activity in composting mixtures. The incorporation of biochar, ranging from 2% to 20% by weight, tends to support larger microbial populations, accelerates the composting process, and produces higher-quality compost in a shorter time frame. Biochar’s ability to create favorable microhabitats within its pores, coupled with nutrient and water provision, contributes to the regulation of microbial diversity and activity. At a macroscale, biochar-amended compost shows improved aeration, nutrient content, pH buffering, and reduced levels of toxic metals, which are beneficial for microbial functions. The fractional contribution of each of the biochar properties to the microbial population in a composting mixture warrants further research. The review concludes with a framework for selecting biochar to foster specific microbial populations, thereby enhancing the overall composting process.