Hydrothermal carbonisation (denoted as HTC) represents a powerful material synthesis platform to convert biomass/biomass derivatives into sustainable carbonaceous and carbon materials. Using a variety of synthetic protocols, functional carbon materials can be synthesised presenting a wide range of morphologies, pore structures and electronic properties. Carbon-based materials synthesised via the HTC platform have already found a broad range of applications including as solid fuels through to electrode materials in fuel cells, batteries and supercapacitors, as water-purification media, CO2-capture materials, soil additives, and as support materials for a range of catalytic species. In this chapter, overview of the HTC process from its discovery until the present day will be provided, with the latest state-of-the-art work by the leading groups in the area specifically highlighted. The chapter will close with a brief introduction to the underlying chemistry behind the formation mechanisms of this exciting class of functional material.