The article reviews the hydrothermal carbonization (HTC) process, a thermochemical method for converting waste biomass into hydrochars, which are carbonaceous materials with potential applications in biofuels, adsorbents, and environmental remediation. HTC operates at moderate temperatures (180–280 °C) using water as a reaction medium, making it cost-effective and eco-friendly. The process involves hydrolysis, dehydration, decarboxylation, aromatization, and condensation reactions, with temperature and residence time being key parameters affecting the structure and properties of the resulting hydrochars. Higher temperatures enhance carbon content and fuel properties, while longer residence times increase yield and aromatic structure. Catalysts, such as organic and inorganic acids, can improve hydrochar formation and functionalization. Hydrochars exhibit superior fuel performance compared to raw biomass, with reduced ash and volatile content, and enhanced energy density. They are also effective adsorbents for pollutants like heavy metals, dyes, and pharmaceuticals, offering potential for wastewater treatment and environmental remediation. The review highlights the importance of optimizing HTC parameters and surface modifications to tailor hydrochars for specific applications, emphasizing their role in sustainable resource utilization and environmental sustainability.The article reviews the hydrothermal carbonization (HTC) process, a thermochemical method for converting waste biomass into hydrochars, which are carbonaceous materials with potential applications in biofuels, adsorbents, and environmental remediation. HTC operates at moderate temperatures (180–280 °C) using water as a reaction medium, making it cost-effective and eco-friendly. The process involves hydrolysis, dehydration, decarboxylation, aromatization, and condensation reactions, with temperature and residence time being key parameters affecting the structure and properties of the resulting hydrochars. Higher temperatures enhance carbon content and fuel properties, while longer residence times increase yield and aromatic structure. Catalysts, such as organic and inorganic acids, can improve hydrochar formation and functionalization. Hydrochars exhibit superior fuel performance compared to raw biomass, with reduced ash and volatile content, and enhanced energy density. They are also effective adsorbents for pollutants like heavy metals, dyes, and pharmaceuticals, offering potential for wastewater treatment and environmental remediation. The review highlights the importance of optimizing HTC parameters and surface modifications to tailor hydrochars for specific applications, emphasizing their role in sustainable resource utilization and environmental sustainability.