Problems of deterioration of cultural and natural property due to changing environmental conditions, ageing, mass tourism, etc. Are discussed; measures for prevention and maintenance (transfer of movable objects to museums, shelters for sites, appropriate microclimate); with two case histories by Roger Lehn: the tomb figure of Jean de Berry (marble sculptured by Jean de Cambrai, 1422-1438), since 1757 in the crypt of Bourges Cathedral, and the inscriptions of the northern belfry of Strassburg Cathedral (commemorative stone plates from 1552 to 1800). 11 figures.