Major porosity-modifying processes operating in the marine diagenetic realm are (1) cementation in shallow warm waters, (2) dolomitization accompanied by minor porosity enhancement in deep waters below the aragonite and calcite lysoclines, and (3) dolomitization associated with organic degradation during early burial. In shallow, normal marine environments, porosity is lost through abiotic and microbially mediated cementation. Such cementation is most abundant in shelf-margin reefs, high-energy intertidal zones, and isolated hardgrounds. Abiotic and microbial cementation in reefs—in combination with bioerosion and internal sedimentation—can totally destroy high initial reef framework porosities. Although intertidal and hardground cements tend to be vertically and laterally restricted, these zones can act as permeability barriers in reservoirs. Deep marine slope and basin environments can experience significant porosity modification. Aragonite cementation on the upper slope extends to 60m depth at present. During the Paleozoic and Mesozoic eras, carbonate mud mounds developed on upper shelf slopes and distal ramps. Microbial processes in these mounds contributed to both early cementation and dissolution. Many such mounds formed in conjunction with hydrothermal and cold petroleum vents on the seafloor; some of these mud mounds are significant reservoirs. Where the thermocline and carbonate lysoclines impinge on steep carbonate platform margins that front oceanic basins, dissolution of aragonite and/or calcite, precipitation of radiaxial calcite cement, and dolomitization can occur. Kohout thermal convection and mixing-zone-induced seawater circulation are the most likely hydrologic pumps driving large volumes of marine water through steep platform margins. At relatively shallow depths in the sediment/rock column, decomposition of organic matter mediated by sulfate-reducing bacteria can promote dissolution of aragonite and Mg-calcite, calcite cementation, and massive dolomitization.