Up until 30 years ago an extensive, flower-rich grassland vegetation containing many species rare in the Netherlands used to be common on Dutch river dikes. However, the deterioration of the flora on dikes was already being reported at the end of the 1960s. At that time too, ecologists warned that the planned reinforcement of the dikes along the Rhine, Waal, Lek and IJssel would adversely affect the flora. Their gloomy forecasts have proved to be correct. Between 1968 and 1992 as much as 89% of the locations with a dry floodplain grassland vegetation in the Netherlands disappeared.In 1992 the vegetation of over 90% of the river dikes consisted of species-poor grassland on which sheep graze, and rough vegetation mown for hay. Only about 7% of the surface area of the river dikes was covered by relatively species-rich grasslands belonging to the phyto-sociological syntaxa Arrhena-theretum elatioris and Lolio-Cynosuretum , both belonging to the Arrhenatherion elatioris . Only 1% was covered by the typical species-rich dry grassland Medicagini-Avenetum . The last remnants of these grasslands are in imminent danger of disappearing.The deterioration in the semi-natural vegetation has mainly been caused by the fact that the slopes of the dikes are increasingly being used agriculturally (fertilization, overgrazing, use of herbi-cides) but also because ecological features were insufficiently taken into account while reinforcing the dikes. In 1984 a research project was started to ascertain the optimum structure and growing con-ditions for the grass cover on river dikes (Sýkora & Liebrand, 1987; van der Zee, 1992). The next step was to test the feasibility of the ecological engineering measures proposed in the above mentioned projects empirically.In the research project described in this thesis the core questions were therefore whether the valuable, species-rich vegetation on the dikes can return after reinforcement works, and, if so, what are the pre-conditions for this during and after the reinforcement. The research ...