Two butterfly species which has become extinct in The Netherlands, Maculinea teleius and M. nausithous, will be reintroduced in 1990. The author, who has been engaged in the preparation of this rcintroduction, gives her views on the problems and solutions surrounding it. Much attention is given to the ecology of both butterfly species and their relationship with the host, the Myrmica ant species. For reintroducrion to succeed, the new site must have the precise environmental conditions for the species to survive. Suitable sites in The Netherlands were found in the province of Brabant. Butterflies will be taken from a stock population in southern Poland.