This special issue contains a series of papers that summarize the research conducted within the European project Urban Dispersion INternational Evaluation Exercise (UDINEE). The main goal of UDINEE is to assess the ability of atmospheric transport and dispersion models used for research and application to predict the consequences of the deployment of radioactive dispersal devices (RDDs). Radioactive dispersal devices, also known as dirty bombs, are explosive devices that can disperse radioactive material in a malevolent and deliberate attempt to harm people and to produce disruption to areas through radioactive contamination. The harmful agent carried by atmospheric transport and dispersion can also be chemical or bacteriological, in which case the release into the environment may be of a different nature than explosive. Andersson et al. (2009) outline the reality of RDD threats and the consequences of such actions. An RDD event would most likely be carried out in an urban environment or, in any case, a scenario that would involve infrastructure and buildings that, when compromised, would create maximum disruption to the societal workings and organization. This implies that if the atmosphere is chosen as the medium for dispersing the agent, complex flows and interactions with infrastructure and buildings are to be expected. The latter constitutes a complex situation for any kind of numerical simulation in either an assessment or prediction mode. In the attempt to anticipate such circumstances, and to set up the optimum environment for emergency preparedness and response, the EU Council in 2009 adopted the European Union (EU) Chemical, Biological, Radiological and Nuclear (CBRN) Action Plan (15505/1/09 REV1). The role of atmospheric transport and dispersion models in assessing and predicting consequences of the dispersion of chemical, biological, radiological agents in complex topography is amply considered in the Plan, and the evaluation of the models' capacity to simulate the contamination and its impact is indicated as a mandatory action. With that in mind, the UDINEE project was proposed to the Directorate General for Migration and Home Affair of the European Commission, which supports the Joint Research Center organizational role in the project. Thereafter the activity was extended beyond the EU research and emergency response context with the direct and active involvement of North