Hydrothermal carbonization (HTC) is defined as combined dehydration and decarboxylation of a fuel to increase its carbon content with an objective of achieving a higher heating value. Temperature is the most important parameter in HTC as it determines the property of water. Various HTC experiments carried out utilize temperature in the range of 180°C to 350°C. The reaction pressure influences the reaction network according to the principles of the LeChatelier principle. High temperature and pressure conditions using only water and biomass is direct HTC while addition of any catalysts to it makes it catalytic HTC. Hydrochar produced through HTC is a high energy density solid product. The heating value of hydrochar makes it competitive with other carbonaceous energy sources like lignite. Hydrothermal carbonization of biomass has great potential to become an environmentally friendly process for the production of a wide variety of products. A number of technologies have been developed to engineer hydrochar to enhance its physiochemical properties.