Due to environmental considerations and the development of new bio-products in recent years, renewable sources as raw materials have grown and, consequently, are attractive both in industry and to the bioeconomy. The abundant availability of biomass as lignocellulosic materials from agricultural residues, forest, pulp and paper industries or from urban solid waste allows for its processing and different applications such as energy, fuels, chemicals, cosmetics, medical applications, construction materials and high added-value products for food or feed, which could be a commercial opportunity. For this reason, there is a clear opportunity to develop commercial processes that could generate products, at very high volumes and low selling prices, from biomass. It is important to consider that the development of efficient integrated processes and technologies for biomass conversion requires the effective utilization of all components. In general terms, and considering all the benefits, the production of high added-value compounds and bioenergy from biomass in the future will have an exponential behavior worldwide. High-pressure processing is a potential clean technology to convert biomass as raw materials into high added-value products and bioenergy. In this technology CO₂–H₂O mixtures, at high temperatures (160–250 °C) and pressures (above 200 bars), are applied for hydrolysis, extraction and structural modification of this biomass. Moreover, this process can be considered an ecofriendly technology, and in terms of a biorefinery, the high-pressure technology as a pretreatment stage plays an important role since the pretreatment allows the fractionation of the main components of the biomass producing high added-value compounds and substrates for conversion into bioenergy. Thus, the use of all products and co-products in the process of pre-treatment is essential, using the concept of biorefinery, which will help reduce the overall environmental and economic impact in the processes. This book discusses the tendency and developments concerning the use of high-pressure processing for the valorization of biomass. It reflects the current state of knowledge of high-pressure technology with an emphasis on fundamentals, chemical proprieties, phase diagrams, thermodynamic proprieties of CO₂, H₂O and the CO₂–H₂O mixture; the development of a sustainable technology in the biorefinery concept, the CO₂ for biomass pretreatment and in the biocatalysis process, the effect of high-pressure CO₂ and CO₂–H₂O mixtures on the direct conversion of biopolymers and as anti-solvent, production of high added-value compounds as furfural, 5-hydroxymethyl furfural (5-HMF), levulinic acid, valerolactone and the perspectives of high-pressure technology. This book will be a useful tool for scientists, engineers and researchers in both academia and industry. The conversion of biomass into high added-value compounds and bioenergy is essential to sustain our present and future and that high-pressure technology is one of the keys to achieving this goal using the biorefinery philosophy.