The European Water Framework Directive (2000/60/EC) commits the Member States of the European Union to classify the ecological status of their surface waters using biological assessment methods. The harmonisation of the good ecological status between Member States is done in the intercalibration exercise. This doctoral thesis comprises the analysis of basic intercalibration approaches and includes results of the first intercalibration phase. In Chapter 1 the direct comparison of invertebrate-based methods is explored. By means of correlation analyses various biological indices are matched for eight countries sharing two common stream types. The outcomes reveal strong relationships between methods, but deviating definitions of the good ecological status. Supportive environmental data is used to illustrate the level of anthropogenic pressure associated with the respective good-moderate boundary of each national method. In search of the most suitable way for comparing national classifications two differing intercalibration options are studied in Chapter 2. The results show that national macrophyte methods are conceptually different: Divergences in the detection of pressures (nutrient enrichment versus unspecific stresses) and the definition of the natural reference state become evident. Chapter 3 identifies the similarities of national methods to establish common grounds in macrophyte intercalibration. The method comparison is enabled by delineating indicator taxa that are used in a common metric for macrophytes. Chapter 4 includes the comparison of ecological classifications for five Eastern European countries. Common metrics are applied in the intercalibration of national methods using benthic diatoms and invertebrates. Since the availability of data from undisturbed reference sites is generally scarce, an alternative approach based on sites impacted by similar levels of disturbance is employed. The biological benchmarks derived from these sites set transnational reference points for the harmonization of national status classifications.