Risk assessment and mitigation again terrorist acts in outdoor Open Areas (OAs) should be based not only on the analysis of possible hazard, physical vulnerability, and perpetrator behaviours and “modus operandi”, but also on joint investigation of the user exposure, vulnerability, and behaviours in emergency conditions. A behavioural design approach relying on the analysis of emergency and evacuation via simulation tools could support these tasks, since it is able to represent complex interactions among these factors and to include users’ reaction and needs to the terrorist event. Indeed, this approach should be supported by quick methods for scenario creation, balancing efforts to manage reliable data and to determine critical phenomena in the OAs. This book offers the definition of a risk assessment and mitigation methodology according to such an approach, applying it to a relevant real-world OA to demonstrate its capabilities in supporting local authorities and their technicians in facing terrorist acts in (over)crowded situations. Different conditions in attack points, users’ exposure and vulnerability and implemented mitigation strategies are derived and tested through simulations, allowing to determine users’ risk levels in emergency and evacuation depending on the combination of such inputs. Nevertheless, the capabilities of the methodology and of its tools should be extended, thus needing further efforts related to both research and practice. This chapter first traces an overview of the main objectives and then suggests future directions for this approach development and application according to both researchers and decision-makers’ standpoints.