The Semmering Base Tunnel in Austria is currently one of the largest tunnel construction sites in Central Europe. Two tunnel tubes, each 27.3 km long, are being built from the portal and three intermediate construction sites. Each access point poses specific challenges, and structural monitoring is a crucial element to ensure both economic and safe construction. In this paper, the authors ... [Show full abstract] describe one of the most comprehensive distributed fibre-optic sensing installations on a tunnel construction site worldwide. At the Semmering Base Tunnel, fibre-optic sensing is used at every construction location to monitor tunnel linings, shafts, reinforced earth structures and pipelines. The authors also discuss the challenges posed by the installations and the results of long-term monitoring programmes. The installed monitoring systems will, furthermore, be used throughout the expected 150-year operational lifetime of the tunnel. Unlike conventional sensors, fibre-optic sensors do not comprise electric or moveable components at the measurement location. A longer lifetime can, therefore, be expected for fibre-optic sensors, which makes them promising candidates for condition-based maintenance of civil infrastructure.