Ensuring the safety and reliability of complex, high-tech systems is of utmost importance. Formal methods allow to check requirements such as “the probability of a system failure within a year is below a certain threshold”. However, applying these techniques requires precise knowledge of the probabilities of component failures.This project aims to develop analysis techniques for systems with unknown or imprecise probabilities. The new techniques allow to ensure that the overall safety requirements are met, even if there is uncertainty in the component failures.