The works for the construction of the 57 km of Transalpine Base Tunnel of the Turin-Lyon railway link involve the installation of outdoor building sites in the Alpine ecological context. The areas affected by these installations have been minimized to reduce the use of soil as much as possible and areas with low ecological value have been chosen as far as possible. The residual impact of the works on habitats, protected species and ecological corridors has been assessed on the basis of detailed inventories. In line with the European doctrine of “avoiding – reducing – compensating”, an extensive program of compensatory environmental measures, covering an area of more than 170 hectares, is being implemented to guarantee a balance sheet without net biodiversity losses. This article describe the compensation program implemented and analyses the similarities and differences in the approach to environmental compensation in the two countries – France and Italy.