In environmental engineering, experts from different disciplines and with diverse backgrounds and responsibilities need to cooperate. As they tackle problems important to society and practice, this type of cooperation can be characterized as transdisciplinary. In transdisciplinary settings, the actors involved bring together theory, models and knowledge from different disciplines. In order to integrate this knowledge and to create a better understanding of the complex issues under consideration, experts can use methods like knowledge mapping. Compared to other methods used in environmental engineering, knowledge maps are based on a lower degree of formalization. In this article, we derive requirements for a method and a tool for supporting the interactive creation of knowledge maps in groups from theory and from existing methods related to environmental engineering. Based on these requirements, we describe the concept of a new method and a supporting tool called DialogueMaps. This tool is web-based and aims at supporting transdisciplinary interactive dialogues for developing multi-layer knowledge maps.