Ash from biomass combustion and biochar produced in the biomass pyrolysis process are becoming important elements of the circular economy and sustainable development. Biochar is recognized as a carbon dioxide removal (CDR) technology that promotes food security, sustainable land management, and the circular economy (Weng and Cowie in Commun. Earth Environ. 6(1):1–9, 2025 [1]). Ash, on the other hand, is an unavoidable by-product of thermal biomass processing and is often still treated as waste stored in landfills—currently, only a small fraction of biomass ash is recycled (Viola et al. in Low-Carbon Mater. Green Constr. 2(1):1–14, 2024 [2]). Meanwhile, both biochar and ash can be reused in ways that bring significant environmental and climate benefits. This chapter discusses how the use of ash and biochar fits into the goals of sustainable development and environmental protection. It presents their role in reducing greenhouse gas emissions and carbon sequestration, their importance in waste management and biomass recycling, as well as the long-term environmental benefits of their use. Particular emphasis is placed on current global trends and research findings in this area.