Nowadays we have all sorts of legislation to safeguard safety at home and at work. Safety management systems are supposed to safeguard safety issues at system level. We have advanced computer models to test system designs when still on the drawing board. Safety as a whole is very much safety conscious. Yet despite all our efforts accidents still happen, unwanted or wanted. Apparently safety as such is not an issue in our modern western society, securing safety is the challenge, hence the title of this thesis. Modern society is increasingly complex and vulnerable. On the other hand our knowledge of risks has increased over the years. Using modern digital risk analysis tools it is possible to design and build structures that would have been impossible only two or three decades ago. Risk management as such is nothing new. The various tools we use depend on the level of knowledge we have about the systems. One thing is clear: accidents should not happen, yet it is difficult to avoid them as this thesis will show. They can be the consequence of unintentional misapprehensions through lack of knowledge and/or understanding of the factual functioning of the system (safety). Or they can be the consequence of intentional acts of destruction such as terrorism (security). A third often used notion is risk: the exposure of a danger or unwanted event. Sooner or later a system can be exposed to one or more threats. These distinction between these notions (safety, security and risk) must be clear. This thesis will use them in several analyses. Damage is unavoidable but should it be fatal? What do we know about the effects when things can go wrong and how reliable is our knowledge about the likelihood of occurrence of such a condition/situation? In most cases the extend of our knowledge is well defined, both about what we know and what we don’t know. It is the knowledge about what we don’t know we don’t know (the unknown risks, the unknown unexpected behaviour of a system following from its hidden properties) that poses a challenge to adequate risk management. How to prepare yourself for the risk you don’t know?@en