In this thesis spin-polarized atomic hydrogen (SPAH) is studied within two complementary approaches. The first is the so-called static fluctuation approximation (SFA) which is used to calculate the thermodynamic properties of SPAH. This approximation is based on the replacement of the square of the local-field operator with its mean value. A closed set of nonlinear integral equations is derived for neutral spin-polarized bosonic systems. This set is solved numerically by an iteration method for a Morse potential. The second approach is the Brueckner-Bethe-Goldstone (BBG) formalism which is applied to SPAH for the first time to calculate the ground-state energy per atom and the depletion.