The group Delphi method enables a knowledge-based and constructive discourse with an interdisciplinary group of experts who present different perspectives and judgments. This textbook introduces the fundamentals of the group Delphi method and illustrates its practical application through concrete research projects. The group Delphi was developed based on the classical Delphi method for the development of knowledge-based interpretations, political visions, and planning tasks, aiming to calibrate interdisciplinary expert judgments. The application of the method in research practice highlights the variety of possible uses and reveals the strengths and weaknesses that arise. A particular challenge is the development of the standardized questionnaire.