Radio astronomy is a passive service and is equipped to observe extremely weak signals from outer space. The sensitivity of current state-of-the-art telescopes is over ten orders of magnitude higher than in most communications systems. Although radio telescopes are best located in relatively remote areas, astronomical observations may be still hampered by man-made radio frequency interference (RFI). In this paper we will consider different interference mitigation options. After a quick overview of techniques already implemented in current radio telescopes, we will focus on the innovative possibilities induced by the new generation of radio telescopes. In particular, phased array architecture can lead to interesting spatial filtering approaches. Examples on real data from LOFAR and from Nançay radio telescopes will be shown.