Composting has emerged as a sustainable solution to address environmental challenges in agriculture by transforming organic waste into nutrient-rich soil amendments. This chapter presents a comprehensive overview of the nature-based ingredients used in composting and their potential to enhance the efficiency, quality, and agronomic value of the final compost. It explores the roles of carbon (C)-rich materials, such as crop residues, woody biomass, and green waste, and nitrogen (N)-rich materials, including green manures, animal manures, and food waste. These feedstocks contribute distinctively to microbial dynamics, nutrient cycling, and structural stability of compost, depending on their composition and carbon-to-nitrogen ratio (C/N). Some case studies using materials such as vine prunings, sugarcane bagasse, olive pomace, and green waste have demonstrated a strong potential to promote nutrient recycling, soil health, and circular agricultural systems while improving compost quality and crop performance. This chapter also highlights the importance of using bulking agents and additives to improve the physicochemical and biological properties of the compost, enhance its maturity and stability, and reduce greenhouse gas (GHG) and odor emissions. Finally, the informed selection and combination of organic residues and conditioners can optimize composting outcomes, mitigate environmental impacts, and support more sustainable farming practices. Future research should focus on regionally available, cost-effective feedstocks and process innovations to expand the role of composting as a climate-smart agricultural strategy.