For intentional acts (terrorist attacks), the uncertainty and knowledge dimensions are much more dynamic than for safety issues. An event occurring on the other side of the earth could quickly change the risk assessment of such acts, the probabilities as well as the strength of knowledge part; the same could be said about the result of surveillance and intelligence work. Clearly, being sensitive to signals and warnings of attacks is essential, as avoiding attacks is of course to be preferred, compared to those relying on the ability of effective barriers to reduce the consequences of the attacks. Robustness and resilience are of course always warranted, but the investment and efforts here have to be carefully balanced against costs and other values are appreciated in a society, such as openness and free movements.