In case of an accident release, radioactive iodine isotope dispersion into the environment is a problem of main concern due to possible human health impact. Authorized discharges result sometimes in measurable levels of radioactive iodine releases into the atmosphere also contributing to public concerns and interrogation – even if at trace level. The half life of some radioiodine isotopes (8 days for 131I and 15 millions years for 129I) implies a possible dispersion into all compartments of the environment – from the troposphere to the terrestrial or marine biosphere and finally into the food chain – inducing potential human health effects. The major accidents of Chernobyl and Fukushima or even the recent iodine release events detected in Europe have shown that the knowledge of the radioative iodine distribution between its various chemical forms is still insufficently accurate to satisfactorily adress the public concerns. Research at IRSN on iodine dispersion in the atmospheric compartment is aimed to adress the major issues of this problematic : 1/ accurate source term evaluation for realistic dose assement, 2/ improved crisis management to better protect the public in case of radioactive iodine release into the atmosphere. To perform these evaluations, IRSN relies on modelling tools dedicated to the evaluation of the source term (ASTEC, PERSAN, ... codes), and to the prediction of its dispersion into the environment taking into account the health impact (C3X, SYMBIOSE and CONDOR tools). Modelling tools are also completed by radioactive iodine monitoring in the environnment (OPERA Air program covering the whole french territory) and mobile in vivo measurement units dedicated to the monitoring of exposed populations. The Software tools allow for an accurate dose assessment. In order to improve its expertise and knowledge on radio-iodine behaviour in the atmosphere, IRSN is currently sustaining large research efforts over 6 thematics covering all the aspects linked to this issue: source term evaluation, atmospheric dispersion modelling including the terrestrial and marine boundaries, development of more sensitive and accurate monitoring equipement, dosimetry tools (ICARE and MIODOSE) and upgrade of prophylaxis policies (PRIODAC research program). Improved understanding of radio-iodine dispersion in the atmosphere in terms of speciation and physical forms can be obtained by combining experimental studies and development of new modelling schemes. Model validation will be gained by facing fields measurements in case of routine iodine discharge ( 129I or 131I to some extent) or accidental/incidental situation. In this view, the development of new field monitoring devices capable of identifying the nature of iodine (gaseous vs particulate material and iodine speciation) is a challenging issue. At least, the dose evaluation operationnal tools and the iodine prophylaxis policy will benefit from the dispersion model improvement. The ability of the nuclear safety field community to accurately characterize radio-iodine dispersion, dose evaluation and progress in the use of countermeasures is thus of utmost importance. Such high accuracy data will help to better advise authorities and emergency crisis managers to improve the protection of the public if a severe nuclear power plant accident is to occur