This article investigates the question of how the significance of potential adverse effects on Natura 2000 sites – comprising sites of Community importance (SCI) and special protection areas (SPA) – can be determined legally and methodologically within the scope of appropriate assessments for projects and plans in accordance with Article 6(3) of the Habitats Directive 92/43/EEC1 (HD) and whether the results can be transferred to the prohibition of disturbance and deterioration stipulated in Article 6(2) HD. The assessment of significance is important as, according to the European Court of Justice (ECJ)2 and the German Federal Administrative Court (BVerwG)3, a project or plan is only permissible if, in the light of the best scientific knowledge in the field and without reasonable scientific doubt, the plan or project will not have lasting significant adverse effects on the integrity of that site. In this process, all aspects of the plan or project have to be identified which may, either independently or in combination with other plans or projects, affect the conservation objectives of the site concerned. This also includes a specialist forecast. Furthermore, closer specification is required of the threshold above which a non-significant adverse effect turns into a significant adverse effect and whether thresholds for bagatelles can be attributed to the proposed development.