In this work, the conversion of lignocellulosic materials in subcritical- and supercritical water was investigated. The degradation of lignocellulose was studied for two reasons. Firstly, plant biomass is a renewable and abundant source of energy and can serve as a feedstock for various chemical products like saccharides and aromatic species. Against the background of a proceeding depletion of fossil materials, the replacement by plant biomass has been gaining increasing attention in the last decades. In order to effectively use plant biomass in future applications, the whole lignocellulosic structure has to be utilized rather than selected parts of plants. For such a sustainable utilization of lignocellulosic biomass, a proper process is still lacking. While the biological degradation is hampered by the fibrous structure and poor solubility of lignocellulosic materials, the chemical fractionation and isolation can only be accomplished by employing organic solvents. The treatment in water at elevated temperatures and pressures could offer an environmentally benign alternative in the utilization of plant biomass due to the specific properties of sub- and supercritical water and the associated interactions with lignocellulosic materials.