Chain growth conducted at substantially elevated hydrostatic pressure is high-pressure polymerization. Pressure is an intensive variable that shifts addition equilibria toward the condensed polymer, increases the rate of bimolecular propagation relative to some termination or transfer steps, and can open reaction channels that are inaccessible at ambient pressure. In conventional vinyl systems, high-pressure radical polymerization of methyl methacrylate and related methacrylates produces poly(methyl methacrylate) glasses whose network density and mechanical response reflect the pressure applied during cure ^[1]. Under still higher, typically gigapascal, pressures, aromatic molecules such as phenol undergo solid-state polymerization to degree-4 carbon nanothreads, in which addition across the ring creates a saturated, thread-like carbon backbone ^[2]. Fused-ring aromatics such as naphthalene likewise polymerize to nanothreads when compressed into an addition-competent lattice ^[3]. The concept is defined by the use of pressure as the controlling thermodynamic parameter of polymerization, whether the mechanism is radical chain growth in a condensed monomer or topochemical addition in a molecular crystal.