Biomass is an attractive source of renewable carbon-based fuels and chemicals and their production is envisaged within the framework of integrated biorefineries. Multiple research efforts to make biorefineries more economically competitive and sustainable are ongoing. In this context the use of high-pressure CO2 and CO2/H2O mixtures for biomass conversion is especially attractive. These mixtures are cheap, renewable, environmentally benign and allow tuning of various processing parameters by varying temperature, pressure and CO2 loading. This chapter presents a broad introduction of the principal processes and conversion routes being considered within biorefineries, and how high-pressure CO2 and CO2/H2O mixtures could help address certain challenges associated with biomass conversion. Some of the principle advantages associated with high-pressure CO2 and CO2/H2O mixtures that we highlight here are their abilities to act as green substitutes for unsustainable solvents, to enhance acid-catalysed reaction rates by in situ carbonic acid formation, to reduce mass transfer-limitations, and to increase access to substrates and catalysts. We discuss these advantages in the context of the trade-offs associated with implementing large-scale high-pressure systems including safety concerns and increased capital costs. With this introduction, we highlight both the principal benefits and challenges associated with the use of high-pressure CO2 and CO2/H2O mixtures, which are further detailed in subsequent chapters.