Strategic Environmental Assessment (SEA) is one of the main tools available to achieve integration of the principles of sustainable development into country policies and programmes, involving a range of analytical and participatory approaches. SEAs are becoming more urgent and necessary but moving from concept to action and towards results has varied. The OECD has produced guidance notes that promote a more harmonised, effective approach to SEA, working alongside key donor and development agencies. The Guidance explains the benefits of using SEA in development co-operation, sets out key steps for its application based on recent experiences, and presents the nine most interesting case studies of SEA in progress.