Recently, our group produced spin-polarized hydrogen (SPH) atoms at densities of at least 1019 cm?3 from the photodissociation of hydrogen halide molecules with circularly polarized UV light and measured them via magnetization-quantum beats with a pickup coil. These densities are approximately 7 orders of magnitude higher than those produced using conventional methods, opening up new fields of application, such as ultrafast magnetometry, the production of polarized MeV and GeV particle beams, such as electron beams with intensities approximately 104 higher than current sources, and the study of polarized nuclear fusion, for which the reaction cross sections of D–T and D–3He reactions are expected to increase by 50％for fully polarized nuclear spins. We review the production, detection, depolarization mechanisms, and potential applications of high-density SPH.