The Eifel maar lakes are of volcanic origin. Because of their favourable geographic location close together, small catchment areas, and great individual differences, they are ideal for limnological studies. From 1910 to 1914 august Thienemann carried out limnological research in the maar lakes. Results from these studies formed the basis for his lake-typology system. He defined the "normal" oligotrophic and eutrophic lake-types and set up a causality chain from the surrounding land via water chemistry and the biological communities to the rest product of the ecosystem, the sediment. Thienemann devoted special attention to the reciprocal relationship between biotope and biocoenosis. These studies, which thus go far beyond and faunistic surveys, became guidelines for the development of limnology. Because of their thoroughness and insight, the investigations carried out by Thienemann offer excellent material for comparisons when describing the development of the Eifel lake ecosystems. The results of continued studies of the Eifel Maar Lakes are therefore presented in this issue.