The high-pressure-related problems of materials constitute a field at the confluence of several scientific disciplines. High pressure can be generated by die compression, high-velocity impact, or explosions. An understanding of materials under extreme high pressure, including flow, plastic deformation, phase transformation, fracture, temperature rise, and chemical reactions, is needed. This Special Issue on "Materials under High Pressure" collected recent research findings on the high-pressure-related problems of various materials. A collection of fourteen peer-reviewed research articles was included in this Special Issue. The main topics covered include processing technology, characterization, testing, theoretic modeling, and simulation.