Uncertainty is a fundamental component of security risk, resulting from inherent randomness (aleatory uncertainty) and incomplete information (epistemic uncertainty). Behavioural research underscores the impact of cognitive biases, including overconfidence and heuristics, on risk perception, despite the fact that traditional risk management models rely on statistical analysis. Furthermore, Douglas and Wildavsky's cultural theory of risk posits that risk is influenced by social and cultural constructs. In this chapter, the intricacies of uncertainty in security contexts are examined, with a focus on the constraints of deterministic models and the promotion of adaptive risk management strategies.