Robustness means many things to many people, even amongst civil engineers. In order to describe its essential aspects more clearly within the context of structural systems and critical infrastructure, the concept of robustness in different but related disciplines has been first examined. Both deterministic and probabilistic or risk-based measures of robustness can be developed and used. In the context of systems subject to exceptional hazards, the uncertainty associated with both systems, hazards and consequences plays a prominent role and therefore measures of robustness must express probabilistically how and to what extent certain system performance objectives are affected by external or systemic perturbations. An example is given for a simple structural system. Structural Engineering International