Observation of ice-stream and ice-shelf flow has revealed interaction with the ocean tide, in some cases far upstream of the grounding line. Ice-stream velocities can be greatly affected by local ocean tides at a variety of timescales but of particular interest is a fortnightly modulation in flow that has been observed on the Rutford Ice Stream. This frequency is absent in the forcing, far stronger than the dominant tidal constituent signals and has been observed far inland. Understanding what mechanism can produce this signal can provide a window into mechanisms at the base of the ice stream that affect its flow. A consequence of this nonlinearity is that the mean flow of the Rutford Ice Stream is increased in comparison to a situation where no tidal forcing is present, implying that changes in tides could alter the long term flow of ice from Antarctica. Since local tides are sensitive to bathymetry which can change drastically due to changes in ice-shelf geometry and grounding line position, a feedback exists between tides and the ice sheet whereby changes in the flow of ice could alter tides which in turn would change the ice velocity. This thesis presents results from both ice-stream/shelf and tidal models to investigate these two mechanisms. Firstly a nonlinear 3D viscoelastic full-Stokes finite element model explores possible sources of nonlinearity that can produce fortnightly modulation in ice-stream flow far upstream of the grounding line. This model looks at stress transmission, grounding zone flexure, hydrology and grounding line migration and the conclusion is that tidally-induced changes in the subglacial water pressure play the largest role in producing this effect. Then, the OTIS tidal model is used to demonstrate that reduction in the thickness and/or extent of ice-shelves, in particular the Filchner-Ronne ice shelf, could greatly impact both tidal amplitudes and dissipation rates in the region. The research in this thesis clearly shows the importance of tidal interactions with the Antarctic ice sheet and that these are not limited to short temporal timescales.