Abstract: Glassy carbon is a nearly fully <italic>sp</italic>²-bonded amorphous carbon allotrope. Its highly disordered atomic structure and isotropic nature make it an ideal model system for studying pressure-induced transitions in amorphous materials and a versatile precursor for the synthesis of novel amorphous carbon materials. This review summarizes recent advances in understanding the structural transformations, property evolution, and transition mechanisms of glassy carbon under high-pressure and high-pressure-high-temperature conditions, as well as its emerging applications in high-pressure science./t/nExperimental and theoretical studies have shown that glassy carbon undergoes a pressure-induced <italic>sp</italic>²-to-<italic>sp</italic>³ bonding transition, forming a tetrahedral amorphous carbon phase with high transparency, electrical resistivity, strength, and bulk modulus. Although this high-pressure phase is not recoverable at ambient conditions, high-pressure-high-temperature treatment can produce new amorphous carbon materials, including compressed glassy carbon and nearly fully<italic>sp</italic>³-bonded amorphous diamond. In addition, the unique nano-pore structure of glassy carbon and its pressure-induced permeability have enabled the development of nanostructured diamond capsules capable of preserving high-pressure phases at ambient conditions. This capability opens new opportunities for high-pressure research and for the practical utilization of high-pressure materials beyond the confines of high-pressure apparatus.