This study addresses the key aspects of the Estonian SEA system and the main factors affecting the effectiveness of SEA. Five dimensions of SEA effectiveness (substantive, pluralist, transactive, normative and transformative) have been examined. The study is also complemented by semi-structured interviews of key actors of the SEA systems (public authorities, SEA experts and environmental NGOs), but also on the authors' own experiences in analysing and participating in the SEA process. The paper concludes that SEA has become a relatively routine process at national government and local government levels, and effectiveness is affected by the level of assessment of cumulative effects (especially those of climate change, biodiversity loss and green area network), consideration of alternatives and extent of public participation. One of the significant impediments to the effectiveness, however, is the missing SEA follow-up.