Hydrothermal biomass conversion processes provide the opportunity to use feedstocks with high water content for the formation of energy carriers or platform chemicals. The water plays an active role in the processes as solvent, reactant and catalyst or catalyst precursor. In this paper, the different hydrothermal processes of carbonization, gasification and liquefaction are introduced and the specific role of water is discussed for each of them. The high reactivity of the polar components of biomass in hot compressed water and its changing properties with temperature are the key to obtain high selectivities of the desired products. Despite the obvious advantages of hydrothermal conversion examples for industrial applications are rare. The main reason for not commercial application of water in the high temperature state is that there are no products that can be sold with profit and cannot be produced cheaper, with less capital risk, and with more simple processes. (C) 2014 Elsevier B.V. All rights reserved.