The Baltic Sea is a shallow, semi-enclosed brackish water basin, which is connected to the North Atlantic via the straits between Denmark and Southern Sweden. It has positive fresh water balance and consequently it is strongly stratified in salinity. The mean salinity is 7.4 per mille, and the strong halocline does not allow vertical convection in the cold season; ventilation of the lower layer therefore takes place by horizontal advection of saltier water. River runoff brings annually a one-meter fresh layer to the basin, while the net atmospheric flux is an order of magnitude less. In winter the Baltic Sea freezes over; the maximum annual ice extent is 12.5–100 per cent of the area of the Baltic Sea and the maximum annual ice thickness is 50–120 cm. The conditions and future of the hydrography of the Baltic Sea are strongly connected to the fresh water inflow. In turn, although relatively small basin, the Baltic Sea influences the weather in the neighbouring land areas, notably the precipitation. This presentation introduces the Baltic Sea, with focus on coastal phenomena and processes, analyses land – sea interaction, and discusses the future of the Baltic Sea and its drainage basin.