Contingency plans for hazards are based on scenarios at different scales. The most extreme scenarios reflect the idea of 'think the unthinkable'. For large-scale floods in the Netherlands, this idea has been given an upper limit called 'worst credible floods': an upper limit for floods that are still considered realistic or credible by experts. Considering the enormous impact of a worst credible flood in the Netherlands and the uncertainty of how a disaster might unfold, a realistic preparation for flood disasters should leave room for improvisation and should be based on relatively simple plans, and on public awareness. The huge consequences of worst credible floods show that the country's safety will continue to depend on pro-active and preventive measures.