The polar sea ice covers greatly affect the energy transfer between ocean and atmosphere and play a key role in the global radiation budget. The physical properties of the ice are controlled largely by the volume fractions and microstructure of the component phases: ice, brine, gases as well as particulate inclusions. Apart from its importance in the climate system, sea ice is of considerable interest as a porous medium that exhibits pronounced temporal and spatial variability in the relevant transport properties as a function of temperature and composition. For Arctic sea ice, liquid phase fractions typically vary between <0.001 and >0.1, while variables such as intrinsic permeability range over 6 orders of magnitude for temperatures between 0 and -25 C. The presentation will focus on linkages between pore microstructure (studied through thin sectioning and magnetic resonance imaging) and transport through sea ice. Specifically, I will discuss (1) how physico-chemistry and microstructure of sea ice control the transition between a low- and a high-permeability regime and (2) how temporal and spatial constraints greatly aggravate the experimental identification of percolation thresholds in natural sea ice.