This chapter describes the compaction effects structure on soil. Soil structure refers to the arrangement of primary soil particles into secondary particles or aggregates. Measurements that describe pore geometry include the water-retention-characteristic curve, permeability/infiltration rate, soil-water diffusivity, sorptivity, air permeability, and gas diffusion. With an increase in the level of compaction, the amount of water held at a high soil matric potential decreases, whereas the amount of water held at a low soil matric potential increases. The shift in water-retention-characteristic curves reflects a change in the relative proportion of soil pores. It is found that an increase in bulk density resulted in an increase in water retention and the magnitude of this effect increased with decreasing matric potential. The range of air content over which the diffusion coefficient is exponentially related to air content decreased with an increase in the level of compaction. This reflected a shift in the proportion of large to small pores during compaction. It is found that the degree of saturation at minimum pore water pressure increases with clay content and then levels off at a clay content between 30% and 40%. The mechanical stress corresponding to a minimum pore water pressure is also elaborated.