In our articles, the topic of criteria for classifying the ecological status of waters often comes up. Have you ever wondered where they come from? Who develops them, why and on what basis? The institution that creates guidelines and methodologies for assessing the ecological status of surface waters in the European Union is the Ecostat Working Group (Ecological Status Working Group), established under the European Commission. It is not widely known, which makes it sometimes confused with Eurostat. As an expert who has been involved in the work of this group for more than a decade, I decided to introduce the scope of its work to the readers of Water Matters.