The description of ocean water masses is based on the study of their temperature, salinity, and density, virtual genetic imprints which provide identity and movement to water masses. Ocean characteristics and processes involved in exchanges with the atmosphere together with simple dynamic balances give an understanding of a large part of the vast oceanic system. This book is enhanced with numerous colored illustrations. It is a reference on regional oceanography updated with extensive results from the last twenty years. The presentation underscores the specificity of each ocean basin using a precise and global approach. Beginning with a brief historical context, it explains the interactions and the role of each ocean basin in the functioning of the planetary ocean. How do we recognize Antarctic Bottom Water in the middle of the Atlantic Ocean? What is the densest water mass? The warmest? Why doesn't dense water form in the largest ocean basin? What becomes of water that sinks in the Labrador Sea? Why does the ocean play such an important role in climate variations? ... Answers can be found in this book. Beyond a course in regional oceanography, the text is aimed at students in all fields of marine and environmental science as well as interested secondary school teachers. It also provides a guide to exploring the "ocean planet" that is comprehensible to any well-informed amateur eager to know the basics.