The resilience of the urban built environment to terrorist acts depends on the interactions among the physical scenario, the attackers, the hosted users, and the mitigation solutions (both structural and non-structural), when implemented. Outdoor Open Areas mainly show a high level of complexity in these terms, and thus, expert risk assessment methods to be applied in such contexts should be also supported by simulation-based approaches, which can be able to manage and describe these interactions in a holistic manner. The behavioural design approach can be used to evaluate the impact of different input conditions on final risk levels depending on the users’ response to the terrorist act. In fact, this approach relies on the experimental-based modelling of user behaviours and individual vulnerability, and on the related simulation in emergency and evacuation scenarios. This Chapter hence traces bases for user behaviour modelling in terrorist acts.