Biocarbons are carbonaceous solids derived from renewable and sustainable feedstocks and their combinations through thermochemical conversion at high temperatures (>350 °C) in the absence of oxygen or in limited oxygen. Expanding their applications from soil and fuel into advanced arenas of polymer composites, energy and environment is the key strategy to substitute for a wide range of conventional fossil-based carbon materials with the added benefits of sustainability and circularity. This Primer discusses biocarbon research, including feedstock selection, characterization, pyrolysis techniques, post-modification strategies, diversified applications and challenges. A critical assessment of carbon sequestration, waste reduction, economic impact, material sustainability and circularity and future perspectives is presented. This Primer mainly focuses on materials (polymer composites), energy (storage and conversion) and environmental remediation (wastewater treatment and CO2 capture). The hurdles that biocarbon-based materials must overcome are effective market propagation, industry-standard adherence and maintenance of a steady flow of feedstocks to guarantee continuous production. Maintenance of reproducibility of biocarbon materials with similar physicochemical and functional properties is another challenging task, which needs more investigation with the support of theoretical modelling and database generation. The Primer also delves into techno-economic analysis, which integrates biomass logistics and their industrial processing, which will enable a new manufacturing platform in biocarbon production for large-scale technological applications.