The research which is presented in the following pages seeks to understand why in some neighbouring countries there had been different short-term responses and different medium/long-term changes following the same event. Particularly it analyses why those in charge were taken by surprise, why different definitions of the situation emerged, why certain limit values for radiation were set and why did they differ, why certain precautionary measures were taken -or not taken- to cope with radiation risk, why different timing in responding to or acknowledging the problems caused by the fallout can be observed. Moreover it inquires why certain changes occurred in the years following the accident, why did they differ or were similar in different countries, and whether they can be interpreted as results of learning processes. In order to provide some ground and context for answering those questions, empirical work (based on individual interviews and qualitative analysis of documents and newspapers) has been undertaken. The field of inquiry was defined according to both analytical and practical criteria. An in depth examination of the events and processes that took place during the last days of April and the month of May 1986 appeared to be necessary to understand why, how and by whom certain decisions were made and why the responses to the same threat varied. Some background information on the main actors involved in nuclear policy and risk management has been gathered and selected to put in context the responses to the Chernobyl fallout. The Chernobyl accident and fallout pointed to serious shortcomings concerning the management of nuclear power both at the national and international level and rekindled scientific disagreements and political controversies over the risks connected with the utilization of such technology. In front of the same event, different responses were given in the affected countries during the fallout and different changes took places in the following years. In my interpretation, this was due to the way scientific and organizational uncertainties were managed in defining the problem and responding to it. The management of scientific uncertainties was a determing factors since Chernobyl had been a technological accident and called on science ability to identify causes, consequences and possible solution of its own technological products. The management of organizational uncertainties was also determinant since an unprecedented, non-routine event like the Chernobyl fallout required the identification and/or negotiation of ill-defined responsibilities. With respect to both these aspects, the production, selection and utilization of information and knowledge by the relevant and interacting actors proved to be crucial. In ‘turn the reason why these uncertainties had been managed in different ways -and different responses to Chernobyl emerged consequently- can be explained in the light of the interaction (in each country and at the international level) between what scientists select as relevant knowledge, what politicians wish to know and to be let known, what pressure social movements and interest groups are able to exert concerning the utilization and diffusion of knowledge, and what information the mass-media have access to, pick out and construct as news. These relations also influenced the changes, and learning processes, that have been taking place since Chernobyl. This hypothesis, which forms the core of what will be called the "Policy Communication Model", points to the link between the ways problems are defined and selected for attention, and the ways actions (including decisions) are taken or not taken. While these two processes had been analysed in depth by several authors, a tendency can be noted to focus either on the first (issue framing) or on the second one (decision making). The present work tries instead to explore the link between the two mentioned processes, building on those contributions that identified such link while still focusing either on the way issues are framed or on the way decisions are made. In the following chapters the changes that occurred in the months and years after Chernobyl will be examined, and it will be argued that they can be regarded as results of collective learning processes. Processes where communication about, reflection on and interpretation (framing) of the Chernobyl experience (which differ 8 for different actors and in different places) played a crucial role in fostering certain changes. Even if these changes do not necessarily represent improvements.