Robustness has recently become a popular term in structural and hydraulic engineering. However, the concept of 'robust design' or 'robustness' has no clear definition. The aim of this work was to find a definition for a robust system and to use this definition to search for an added value of robust design in structural engineering. The literature on robust design has been extensively studied to es-tablish a definition for a robust system. It was concluded that a robust system can be found by as-suming that the additional risk that arises due to discrepancies between the reality and the model, is smaller for a robust system than for a less robust system. When designing a system, it is however al-ways the priority to find a cost-optimal design alternative. In other words, economic optimization has priority over robustness. The reason for this it that robustness focuses on hazards for which no value of the risk can be defined, so that it cannot be quantified what may be spent on robustness. In this thesis, the meaning of a robust system was tested in several case studies. From the case studies it as investigated when and why robustness can be of added value.