Water can have many different roles in catalytic biomass-based technologies to produce chemicals and fuels. Since water is one of the two basic chemicals of nature used during photosynthesis, biomass contains different amount of water, which can be advantageously utilized during the conversion schemes. Water could be the solvent of the reactions or just either the eliminated product or the reactant resulting in hydration and hydrolysis. Since some of these roles are intrinsically part of the natural system, their change could be very difficult or not even possible. Therefore, the designer of the chemistry or the process must understand the possible roles of water at both the molecular and the macroscopic levels. (C) 2014 Elsevier B.V. All rights reserved.