This paper addresses contributions that dynamic geometry systems (DGSs) may give in reasoning by contradiction in geometry. We present analyses of three excerpts of students' work and use the notion of pseudo object, elaborated from previous research, to show some specificities of DGS in constructing proof by contradiction. In particular, we support the claim that a DGS can offer guidance in the solver's development of an indirect argument thanks to the potential it offers of both constructing certain properties robustly, and of helping the solver perceive pseudo objects. [This paper was originally published as Baccaglini-Frank, A., Antonini, S., Leung, A., & Mariotti, M. A. (2011). Reasoning by contradiction in dynamic geometry. In B. Ubuz (Ed.), "Proceedings of the 35th Conference of the International Group for the Psychology of Mathematics Education" (Vol. 2, pp. 81-88). Ankara, Turkey: PME.]