The thesis addresses the introduction of a supra-national instrument; a European directive on Strategic Environmental Assessment (SEA) into national contexts of land-use planning in three countries; Sweden, Iceland and England. The directive ”On the assessment of the effects of certain plans and programmes on the environment” was agreed upon by the European Commission on the 21st of June 2001 and was to be transposed to national legislation by 21st of June 2004. The introduction of these requirements meant that the countries needed to make legal adjustments and implement it at the different levels of planning. Many EU member countries, including those studied in the thesis, had some experience of environmental assessment of plans and programmes prior to the introduction of the SEA directive. SEA has as a concept and a tool in planning in national and international debate on Environmental Assessment and planning for the last two decades. Hence, the SEA directive was introduced to an existing context of environmental assessment in planning and the preparation of the directive has drawn on substantial conceptual development and practical experience of strategic environmental assessment in various forms. The aim of this research is to shed a light on the transposition of the SEA directive into a national legal framework and how the introduction relates to the countries’ planning contexts and previous application of SEA-like instruments. In the thesis an overview is given of the way the directive is transposed to the national legal system of the three countries and the existing planning framework is described. The results from the national reviews are analysed in relation to the contents of the directive and the international and Nordic academic debate regarding the purpose and role of SEA, related to the characteristics of the planning system. The research shows differences in the legal and planning contexts to which the SEA requirements have been introduced in the three countries. Despite of those, the legal requirements follow closely the contents of the directive. However, the expectations towards the directive expressed by national officials and politicians, the recommendations in the way the legal SEA requirements shall be implemented, differ between the countries as well as references to other processes; land-use planning and the practices of Environmental Impact Assessment and Sustainability Appraisal. The thesis is the result of a project within the interdisciplinary research programme MiSt, “Tools for environmental assessment in strategic decision making” at BTH funded by the Swedish Environmental Protection Agency. The project has been carried out at Nordregio, the Nordic Centre for Spatial Development, Stockholm.