Many ecologists have occupied themselves with the problem of the littoral zonation and they still disagree among themselves about the delimitation of the zones and the factors which cause the zonation. In this paper the problem has been approached in two different ways. In the first place the littoral zone sensu lato has been regarded as the border area between the sea and the land. This approach does not result in an explanation of the ‘submergence’ of many eulittoral organisms in the Arctic waters and the Baltic. Secondly the littoral zone sensu lato has been considered as the border zone between the sea and the fresh water, in other words as a brackish environment. This approach gives a reasonable explanation for the ‘submergence’ of littoral organisms in low-salinity waters. It appears that both tidal movements and wave action (wash, splash, and spray) cause salinity fluctuations and that salinity is one of the major factors acting in the littoral zonation. Most of the organisms which do not show ‘submergence’ in low-salinity waters are restricted to the supralittoral and the upper part of the intertidal zone, where they are beyond the influence of the daily tidal rhythm. The vital factor for these organisms is contact with the air. The third active factor, light, is of some importance in the lower part of the eulittoral. These three abiotic factors, together with biological stress (competition), effectuate the littoral zonation pattern. Both the supralittoral and the eulittoral are well-defined ecological units, characterized by their own physical features and their own species composition. In fact they have almost no common species. Strong wave action allows some eulittoral species to survive in the supralittoral; this has to be regarded merely as an interference phenomenon. The littoral border environment is in comparison with other brackish habitats extremely rich in species peculiar to it. In this environment the influence of the salt extends far above the tidal influence. In the estuarine environment, which has very few species exclusively bound to it, the reverse is the case. This could indicate that marine salt beyond the reach of the regular tidal rhythm creates conditions favourable for speciation.