The European Union and its Member States have been creating a network of protected areas for habitats and species since 1979. In 2017, this included over 27,500 Natura 2000 sites, a combined area of over 18 percent of the land surface in the EU and around 395,000 km2 of marine territory. According to Article 6(3) of the Habitats Directive 92/43/EEC1 (HD), any projects and plans within these sites or in their vicinity require an appropriate assessment to ensure that they will not have a significant impact on the integrity of Natura 2000 site. The project or plan is to be rejected by the national authorities if this cannot be excluded without remaining reasonable scientific doubts. This article explores the procedural steps and the requirements that must be examined, which are now covered by comprehensive European Court of Justice (ECJ)2 case law. Numerous questions that are relevant in practice, however, have only been considered by national courts to date. These will be introduced in this article based on the decisions of the German Federal Administrative Court (BVerwG)3 and will be the focus of a critical discussion. Questions on the range covered by the term project and on determining significant impacts will be entered into in greater detail in two further articles, given the scope of the aspects to be examined and explored.