Understanding the gas phase flow and transport of volatile organic compounds (VOCs) in unsaturated zones is indispensable to develop effective environmental remediation strategies, to create precautions for fresh water protection, and to provide guidance for land and water resources management. Atmospheric pressure and water table fluctuations are two important natural processes at the upper and lower boundaries of the unsaturated zone, respectively. However, their significance has been neglected in previous studies. This dissertation systematically investigates their influence on the gas phase flow and transport of VOCs in soil and ground water remediation processes using analytically and numerically mathematical modeling. New semi-analytical and numerical solutions are developed to calculate the subsurface gas flow field and the gas phase transport of VOCs in active soil vapor extraction (SVE), barometric pumping (BP) and natural attenuation taking into account the atmospheric pressure and the water table fluctuations. The accuracy of the developed solutions are checked by comparing with published analytical solutions under extreme conditions, newly developed numerical solutions in COMSOL Multiphysics and field measured data. Results indicate that both the atmospheric pressure and the tidal-induced water table fluctuations significantly change the gas flow field in active SVE, especially when the vertical gas permeability is small (less than 0.4 Darcy). The tidal-induced downward moving water table increases the depth-averaged radius of influence (ROI) for the gas pumping well. However, this downward moving water table leads to a greater vertical pore gas velocity away from the gas pumping well, which is unfavorable for removing VOCs. The gas flow rate to/from the barometric pumping well can be accurately calculated by our newly developed solutions in both homogeneous and multi-layered unsaturated zones. Under natural unsaturated zone conditions, the time-averaged advective flux of the gas phase VOCs induced by the atmospheric pressure and water table fluctuations is one to three orders of magnitude less than the diffusive flux. The time-averaged advective flux is comparable with the diffusive flux only when the gas-filled porosity is very small (less than 0.05). The density-driven flux is negligible.