Applied limnology deals primarily with the effects of stressors on freshwater ecosystems and with measures for mitigating stressors and restoring degraded water bodies. It contributes to the development of methods for the assessment and restoration of freshwater ecosystems, but also to the refinement and implementation of policies, in Europe in particular to the implementation of the Water Framework Directive (WFD). According to this directive, the target of water management is not only good water quality, but “good ecological quality”, measured as the distance of freshwater biota to undisturbed references. Working groups of the University of Duisburg-Essen have greatly contributed to develop assessment methods measuring ecological quality, which are based on profound knowledge of how individual freshwater species respond to a variety of stressors, including various sources of pollution, habitat degradation and flow alteration. The data are summarized in the online database www.freshwaterecology. info. Currently, the methods are undergoing a major shift from morphology-based species identification to automated DNA-based identification, which is driven by the EU-funded COST action DNAqua. Net, coordinated by the University of Duisburg-Essen. According to the assessment systems’ results, about half of the European water bodies are currently not reaching “good status”, but the causes of degradation differ regionally: while in Northern Europe the effects of water power generation and remnants of acidification are predominant factors, Southern European water bodies are particularly affected by water scarcity caused by irrigation. In Central Europe, many different stressors at medium intensity affect water bodies simultaneously, including input of nutrients, pesticides and fine sediment and, in particular, hydromorphological degradation. To restore degraded water bodies, a large number of restoration projects have been initialised over the last couple of years, the effects of which were intensively studied by scientists of the University of Duisburg- Essen. The effects of restoration measures on freshwater biota vary, with floodplain organism groups (e.g. riparian ground beetles) responding most rapidly, while the response of most aquatic organism groups is delayed. Using a paired study design, it could be revealed that the length of the restored section does not greatly impact the restoration success, but the amount of habitat changes on the river bottom is of great importance. Recolonization of restored river reaches with sensitive organisms is mainly based on colonization sources in the immediate surroundings (up to five-kilometer distance). A detailed study on recolonization was performed in the Emscher catchment, which offers great opportunities for studying the succession of newly established freshwater assemblages.