Explore the intrinsic complexity of the oceanic system in “Ocean in the Earth System,” part of the Ocenography and Marine Biology Series: Seas and Oceans Set. Authored by Patrick Prouzet, this first edition delves into the interactions between the ocean and its environment—such as the atmosphere, geosphere, and biosphere—and how these exchanges impact energy and matter transfers. The book examines hydrodynamic mechanisms at various spatial and temporal scales and their role in biogeochemical cycles and tracers. With a pedagogical approach, it introduces key terms, methods, techniques, and analytical tools essential for understanding oceanic complexity. It also explores the consequences of climate change, future projections, human impact, and highlights the planktonic pelagic ecosystem component within this intricate framework.