Biochar is the carbon-rich organic matter that remains after heating biomass under minimization of oxygen during a process called pyrolysis. Biochar is relevant to deforestation, agricultural resilience, and energy production, particularly in developing countries. Biochar Systems for Smallholders in Developing Countries: Leveraging Current Knowledge and Exploring Future Potential for Climate-Smart Agriculture reviews the opportunities and risks of biochar systems and discusses the current knowledge of biochar science. The report also offers a reconciling view on different scientific opinions about biochar, showing the various perspectives of its science and application. This includes soil and agricultural impacts of biochar, climate change impacts, social impacts, and competing uses of biomass. The authors contextualize the current scientific knowledge so that it can address the development–climate change nexus, including social and environmental sustainability.