Abstract: The aquatic fern Marsilea quadrifolia L is a rare and threatened species in entire Europe due to wetland habitats destruction and changing agricultural practices. To protect it, in situ and ex situ conservation methods are approached in European Union an in other countries. The in vivo and in vitro collections that were developed in botanical gardens in the last two decades are used for reintroduction and for restoration of M. quadrifolia populations in natural sites as well as in agro ecosystems that are analogous to natural habitats. Natural establishment of several M. quadrifolia populations in its natural range is an evidence that it can colonize new suitable habitats, including anthropogenic habitats. Despite conservation strategies approached within the European Union, its area of occupancy has decreased, thereby this species has become vulnerable at European Union level. The main threats are the small size populations, low genetic diversity and genetic erosion of populations, habitat degradation and chemical pollutions of waters by herbicides and fertilizers used in modern agricultural practice.