In this preface, we present an overview of the topics and scope of the European Conference on Modelling Foundations and Applications (ECMFA) and describe the editorial and reviewing process for its 21st edition (ECMFA 2025). We also provide an overview of the papers selected for publication and presentation, and details about the keynote talks by Anne Koziolek and Matthias Volk. Finally, we acknowledge the work of the ECMFA committees.