Since the appearance of the computer geometry program Cabri Géomètre in 1988, Dynamic Geometry Environments (DGE) have shown themselves to be useful tools for doing work in elementary geometry. From the beginning, it has been clear that these were tools for the teaching and learning of geometry as well. Thus, high expectations surrounded their heuristic potential to aid in problem solving, for inductive proofs, etc. In this thesis, I am investigating two research questions: (i) What are the effects of DGE on a learner's understanding of geometrical proofs? (ii) How do they use the drag mode and what cognitive benefit do they obtain from their use of it?The learners for this study were mathematics teacher students at the University of Paderborn. Their interaction with the DGE was observed and recorded within the framework of semi-structured interviews according to Mayring; afterwards the transcriptions from the interviews were evaluated based on the principles of Objective Hermeneutics according to Oevermann. The results found were the following: (i)Those students with an inappropriate understanding of the nature of a mathematical proof tend to believe in an implicit ability of the DGE to prove statements - an ability which is known to be non-existent. (ii) Those students with little or no experience with the drag mode display technical cognitive (!) problems in using it. In general, the results of earlier studies have been confirmed, according to which the undeniable didactic potential of DGE will in no way realize by itself.