The hydrogeologic properties of igneous ocean crust have been tested directly in only a few locations during IODP, but more common studies of crustal structure and rock alteration (using core samples and wireline logs) provide insight as to how water–rock interactions modify the crust over time. Collectively these studies reveal strong lithologic and hydrogeologic control on the nature of water–rock interactions, with hydrogeology following crustal architectures and histories. Permeability is generally greatest in the upper crust, but is heterogeneously distributed with depth and (at least in one location) may be azimuthally anisotropic. There appears to be a spreading rate dependence of basic patterns of rock alteration in the upper oceanic crust, with more variable and extensive alteration observed in crust created at slow- and medium-rate spreading centers. There may also be a spreading rate dependence of hydrogeologic properties, but we currently lack direct observations to test this hypothesis. The evolution of crustal properties with age is consistent with sustained ridge-flank water–rock interactions, and a continued dependence on fluid flow rates and reaction temperatures.