Publisher Summary This chapter discusses the physical oceanographic conditions in the Baltic Sea, including the salinity and temperature distribution, and their seasonal and long-term variations. The chapter also discusses the density stratification and its variations, the mean and time-dependent circulation, and its relation to wind and thermohaline effects. The vertical and horizontal transports generated by different mixing processes in the open sea and in the coastal boundary zone and the optical conditions and aspects of the heat budget and ice conditions are also discussed in the chapter. The physical oceanography of the Baltic Sea is governed by the water balance—that is, the fresh-water supply and the water exchange through the Danish Straits. The topographic conditions are also of great importance, particularly, the division of the Baltic Sea into several basins separated by more or less well-defined sills, and the shallow and narrow connections with the North Sea.