Non-native marine organisms may strongly affect resident species communities and interactions. Therefore, non-native species are an important criterion within the European Marine Strategy Framework Directive (EU-MSFD) to evaluate environmental status. The goal is to achieve a good quality for all European seas by 2020. In order to attain this goal, Germany developed a survey programme for nonnative marine species and an assessment system in close cooperation between national authorities and science. These recommendations are accepted internationally, and are currently being harmonised. During this process, it became apparent that the issue of non-native species is regularly affected by subjective criteria.