The determination of the location of a leak in full coverage tunnel waterproofing systems represents an important task during the investigation of existing double-shell tunnel linings. The exact determination of the location of a leak provides several advantages during the planning and realisation of repairs. These include the optimization of construction procedures by enabling selective repairs as well as improved estimations of costs and schedules. Destructive investigations to determine the location of a leak are usually not practical or economical. Reliable non-destructive methods are therefore in demand. Thermography has been applied for several years within the framework of condition assessments in tunnels. Successful application have been documented in locating the boundaries of problem areas of structures as well as in the localization of existing points of water inflow, damp areas and water behind the lining. Within the scope of the proposed research project, the question of whether portable thermographic cameras are suited for the precise determination of the leakage point in double-shell underground structures is to be investigated.