The hydrography and dynamics of the Baltic Sea, although ruled by the same principles and forcing factors as any part of the World Ocean, contain several distinguishing features. Apart from the complicated geometry and bathymetry of the basin, two major factors contribute to the complexity of the processes here. The interplay between inflowing saline, dense waters from the North Sea in the bottom layer with the excess of light, and fresh riverine waters coming into the system in the upper layer leads to the formation of a permanent two-layer structure of density separated by a sharp jump layer (halocline). Due to the layered structure, the direct atmospheric forcing is restricted to the upper layer with a typical thickness of 40–80 m, while in the bottom layer advection and mixing processes govern the patterns of the hydrographic fields. On the top of the upper layer, a well-mixed surface layer, with a typical thickness of 15–20 m, is formed due to summer-time heating, whereas at the bottom of this layer a rather sharp jump layer of temperature (thermocline) exists. During autumn the vertical temperature gradient vanishes due to thermal convection and turbulent mixing. There are four mechanisms which induce currents in the Baltic Sea: the wind stress at the sea surface, the surface pressure gradient, the thermohaline horizontal gradient of density and the tidal forces. The currents are steered furthermore by the Coriolis acceleration, topography and friction, forming a general (cyclonic) circulation in this stratified system with positive fresh water budget. Due to the shallowness of the Baltic Sea, bottom friction damps the currents remarkably. Voluminous river runoffs can produce local changes in the sea level height and consequently also in currents. Inflowing waters penetrate at depths where the density of the ambient water matches the inflowing water masses. Due to the small baroclinic Rossby radius (2–10 km), the proper descriptions of mesoscale eddies, fronts and mixing processes need high-resolution modelling.