For tunnel, it is customary to base the decision for a new construction, a rehabilitation or modernisation on the investment costs only. The very high follow-up costs caused by the requirements for safety, durability and availability are not taken into consideration. For the implementation of sustainability, the total costs of the life-cycle of a tunnel are to be captured in a transparent way. At the same time, it is important to optimise them. The concept of life-cycle costing, the standard concept in many areas, provides a suitable approach and is adapted to the specifications of tunnels. For the implementation of a holistic approach, a life-cycle cost model is developed, which enables an equal inclusion of initial and follow-up costs. In order to develop a clear and transparent approach for the determination of the life-cycle costs, the tunnel is structured hierarchically. To achieve the additional requirement of flexibility and the opportunity to generate optimisations of the life-cycle costs, the hierarchical structure has been extended by modularisation. Thereby modules represent largely independent units, which allow an entire representation of the object of investigation. In order to ensure comparability and transferability a general valid internal structure is defined in the form of attributes. Additionally the variety of the services of each module requires a classification in superior processes, assigned to the associated life-cycle phase. The resulting Modular-Process-Model forms the basis for the determination of the life-cycle costs and for the selective generation of optimisation approaches. For a transparent determination of the life-cycle costs, a self-contained methodology has been developed, based on the Modular-Process-Model. Taking the internal and external interfaces into account, as well as the knowledge of relevant influence parameters enables a selective adjustment or substitution of modules, processes or attributes for a holistic economic optimisation. The dependence on future conditions means that the data for a cost forecast are subject to uncertainties. For an integration of resulting risks the Modular-Process-Modell can optionally be extended by implementing probabilistic input parameters. The determination of the life-cycle costs is carried out by the integration of a risk analysis in an extended methodology using Monte-Carlo-Simulation. The application of the developed approaches is not only limited to the design phase. The Modular-Process-Model, with its implemented consistent cost structure, forms the basis for a continuous capture and verification of the costs throughout all life-phases. The comparability with similar structures, modules or processes creates the possibility of a continuous generation of optimisation potentials. Therefore the concept of benchmarking was adapted to the objective and integrated in a self-contained approach. The functionality and applicability of the Modular-Process-Model and the developed methods are verified by an exemplary tunnel. Besides, the flexibility and transparency could be demonstrated by the modular and process-related design as well as the possibility for the generation of optimisation attempts – also using the benchmarking method. It could be shown that the developed approaches allow a transparent, holistic capture of the life-cycle costs of tunnels, influenced by the objective and the available density of information. The integration of interdependencies enables the targeted optimisation of modules and processes. Due to the interaction of the developed methods and the embedded Modular-Process-Model, the targeted economically optimized design of tunnels can be realized.