Hydrothermal carbonisation is a thermochemical conversion process that transforms organic feedstocks into a carbon-rich solid product (hydrochar) under subcritical water conditions, typically involving moderate temperatures (180-250°C) and autogenous pressure. As an academic concept and methodological approach, this field investigates the intricate reaction mechanisms, optimal process parameters, and product characteristics (hydrochar, liquid, and gas phases) to enable the valorization of diverse biomass and waste streams for applications in renewable energy, sustainable materials, and environmental management.