Consistent scenario thinking is the key to effective consequence control. In these scenarios the consequences of any release of a liquid, gas or solid in space and time for people and environment shall be evaluated, irrespective of the probability of occurrence of such a release. These scenario studies will yield the criteria a storage system has to be designed for. Basically we can distinguish between three tightness classes, viz.: • Full tightness • Apparent or technical tightness • Adequate tightness The main characteristics of these tightness classes, and how the required tightness can be realized, are listed in Table 1. Some additional comments follow below.