The production, destruction, and preservation of CaCO3 in the oceans are important processes that directly affect major pathways and reservoirs of the global carbon cycle and, thus, are strongly linked to global climate and environmental conditions. Since the industrial revolution, the oceans have absorbed a significant proportion of the CO2 released to the atmosphere from human activities. As a result, the seawater acid–base balance has changed in favor of increasing acidity (decreasing pH) and decreasing saturation state with respect to CaCO3 minerals, and will continue to change as long as CO2 emissions to the atmosphere keep rising. These ongoing changes in seawater chemistry have collectively been referred to as ocean acidification. Because the production, destruction, and preservation of CaCO3 are strongly coupled to seawater saturation state with respect to CaCO3 minerals, continued ocean acidification will undoubtedly impose changes to the contemporary oceanic CaCO3 cycle. The production of CaCO3 mainly by calcifying marine organisms could decrease while the destruction by dissolution and bioerosion of carbonate particles, sediments, and structures could increase. This chapter reviews the major processes responsible for the production and destruction of CaCO3 in the oceans and how these processes will change in response to rising atmospheric CO2 and ocean acidification.