The concept of water masses is reviewed from the point of view of quantitative water mass analysis. A theoretical framework is presented which describes the life history of water masses in terms of formation, consolidation, aging and decay. Water masses are described as physical entities and compared with their atmospheric counterparts (air masses). The classical temperature-salinity diagram is expanded into the mathematical concept of water types in an n-dimensional parameter space. Water types and their standard deviations are introduced as the foundation for quantitative water mass analysis. The relationship between parameter space and physical space is established through the definition of water type density. Mode Waters are discussed as regions in physical space with a minimum in water type density. Some unresolved issues of the structure of the oceanic thermocline are discussed in this context. The definition of water masses is extended to include water masses in the surface mixed layer where air-sea exchange processes continuously modify water mass properties. The paper concludes with a brief discussion of the representation of water masses and their evolution in numerical models.