The study of reactions involving liquids at elevated temperatures and/or pressures is important in a wide range of disciplines, including earth and planetary sciences, corrosion, materials, geo-engineering and biological/environmental studies. In situ X-ray absorption spectroscopy (XAS) measurements have had a profound impact on the field by providing direct insights into molecular-level reaction mechanisms at elevated pressure and temperature. This chapter summarizes the various approaches underpinning the design of reaction cells suitable for the study of these reactions under extreme conditions via XAS methods.