Ice is one of the most ubiquitous materials in our solar system, appearing on nearly every planet and satellite. Moreover, the mechanical properties of ice exert a first-order influence over various geophysical processes within these diverse planetary settings. Where ice is sufficiently thick, hydrostatic pressures suppress fracture, and the viscous deformation of ice prevails even at relatively high differential stresses. By contrast, most ice deformation experiments are performed at ambient pressure and are corralled to a limited range of stress and strain that is not representative of the full diversity of structures and behaviors present in natural settings. In this dissertation, ice deformation experiments were conducted on ice at high confining pressure to provide more robust datasets to better model ice flow at stresses and strains which are typically inaccessible in ambient pressure experiments.In Chapter 2, we investigate the influence of grain-size on ice flow at high stresses. Deformation of ice at glaciologically high stresses (≥ 0.1 MPa) is predicted to be insensitive to grain size. However, we report that flow rate is inversely correlated with grain size by a power of 1 to 2 even in a putative dislocation creep regime. In Chapter 3, we report new experimental constraints on activation energy, Q, which describes the sensitivity of ice flow to temperature. We observed a consistent increase in the strain rate with temperature that results in a convex-upward curvature representing an apparent value of the activation energy on an Arrhenius plot of log strain rate vs. 1/T for temperatures exceeding ~255 K to nearly the melting point. Convention treatment of activation energy in ice flow models by best fit linear regressions above and below a temperature threshold systematically underestimates ice flow at temperatures approaching the melting point. In Chapter 4, we perform and interpret results from forced oscillation experiments at previously unexplored high stresses to constrain the dissipation of mechanical energy in ice. Icy planetary bodies in the outer solar system dissipate mechanical energy from tidal interactions as heat, generating an internal ocean or driving plate tectonics in their icy shells. Tidal heating models rely on linear viscoelastic models, but above some critical amplitude, dislocation motion results in nonlinear dissipation. We observe amplitude dependent attenuation consistent with nonlinear viscoelastic behaviour.