Permeability of hardened cement paste (hcp) was studied in relation to the modes of change in pore structure due to hydration and water/cement (w/c) ratio. The effects of the addition of aggregate to hcp on permeability of mortars were studied in this thesis. The saturated permeahility was measured by a steady-state method which takes advantage of the do$tc prSJrC in sealing truncated conical disc specimens at pressures up to 2000 psi. Complementary measurements of the pore size distributions by mercury porosimetry, total porosities by helium comparison pycnometry and the evaporable water contents at 105°C were made to characterise the pore structure of hardened cement paste and mortars. The purpose was to identify the most appropriate variable(s) of the structure of hcp and mortars that relate(s) to permeability. It was found that although the permeability of hcp increases with increasing total porosity, permeability is a multivalued function of total porosity and depends on whether the change in porosity derives from changes in water/cement ratio or times of hydration. It was possible to identify, from the pore size distributions, a primary continuous pore radius, which is thought to correspond to the maximum spacing between the weakest links in the bonds of cement hydration products. It was found that permeability of hardened cement paste was closely and uniquely related to the primary continuous pore radius irrespective of whether the mode of change in pore structure results from differences in water/cement ratio or times of hydration. The measurements of permeability of mortars prepared with both normal and lightheight aggregates indicated no substantial differences in permeability for the to types of aggregate inclusions in the pastes. It was found that the addition of aggregates of lower total porosity than the paste to the pastes reduce the total porosity of mortars but increase permeability as the volume concentration of the aggregate increases. Suggestions for future work include further measurements of the effect of aggregate additions to pastes at different levels of paste porosity on permeability. Measurements to elucidate the fundamental relationship between the primary continuous pore radius and compressive strengths and elastic constants of hardened cement paste, and their relationships with the nature of the cement hydration products.