Carbon is a fundamental component of all life and its gaseous form, carbon dioxide (CO2), is an important atmospheric regulator of Earth’s climate. Over societally relevant timescales, diverse physical, biological, and chemical processes in the global ocean have a very important role in controlling the exchanges of CO2 with the atmosphere and climate, given that the ocean contains roughly 60 times more carbon than the atmosphere. The complex and dynamic marine carbon cycle is thus a fundamental part and regulator of life on Earth, requiring monitoring of its variability and understanding of the feedbacks to the climate system. This is particularly relevant given that emissions of human produced (anthropogenic) CO2 through fossil fuel and land use changes have led to significant perturbations in the global carbon cycle. A large fraction of emitted anthropogenic carbon (Cant) has been and will be absorbed by the oceans with implications for seawater chemistry, ocean pH levels, and the biological communities contained within. During the last few decades, quantification of the ocean sink of CO2 and its temporal evolution of the marine carbon cycle has been an important driver of marine biogeochemical research. Scientific expeditions such as the World Ocean Circulation Experiment/Joint Global Ocean Flux Study global surveys conducted during the 1990s provided the data for the first global estimates of the uptake and oceanic sink of Cant. More recently, completion of a second global survey of the marine carbon cycle (from 2000 to 2013) provided sufficient data to assess the rate of uptake and storage of Cant within the interior of the global ocean. From this effort, it has become clear that there is substantial regional and temporal variability of the storage rate of Cant and those physico-biogeochemical processes that influence the marine carbon cycle. Here we review seawater carbonate chemistry, the sampling strategies, and networks required to observe variability in the global ocean carbon cycle and the flux of carbon between the ocean and the atmosphere but with a central focus, synthesizing and summarizing available estimates of the ocean uptake and inventories of Cant. This chapter is aimed at a wider audience within the oceanographic community and points to the rich literature on marine carbon cycle research.