Up to the present the water-plant communities have been studied in the same way as the terrestrial plant communities. As water and land differ fundamentally as a habitat for plants it is not surprising that several ecological concepts, which have been developed for the terrestrial vegetation, can not be applied to the aquatic vegetation. In the water quite other factors are decisive for the development of communities than on the land. The system of Braun-Blanquet, normally used for the classification of plant communities, is based on one complex character, the floristic composition of the vegetation. Although this character is highly estimated by us, we think, nevertheless, that a one-character system is of necessity artificial. To arrive at a more natural classification other criteria have also to be considered. The floristic composition of the vegetation on its own appears to be an insufficient character for the classification of the water-plant communities, as a consequence of the equalizing effect of the aquatic medium. In the system proposed the following characters have been applied: floristic composition, life-form spectrum, physiognomy, stratification and in some cases the ecology of the vegetation.