This paper reports part of a design research experiment aimed at studying the influence of Dynamic Geometry Systems (in particular, GeoGebra) in students´ mathematical competencies development. It was conducted in two cycles with four classes at a secondary school. The results show evidences that the software helped students to develop mathematical competencies at different degrees. Geogebra provided students enough support for reaching a good level of performance in competencies related with using tools, managing representations, and problem solving. The software supported students in achieving basic to medium levels in competencies related to reasoning, argumentation and communication. In these last competencies, social interaction proved to be a key factor for students´ progress.