Gas-expanded liquids and near-critical fluids are tunable solvent media for conducting chemical reactions, separations, and materials processing. In recent years, there has been an expanding body of fundamental and applied research in the use of these media, particularly for applications to sustainable processes based on the principles of green chemistry and engineering. This volume spans research and development activities ranging from theoretical and experimental investigations of the fundamental science underlying the field to developmental aspects of the technology for commercial applications. An introduction and overview of the field is included, as well as both new developments and reviews organized in the general topical areas of thermodynamics and transport properties (including both experimental and modeling studies), reactions, and materials processing. To the best of our knowledge, this is the first such compilation in this field with contributions from leading research groups.